These neglected plot threads significantly influenced the evolution of the characters and the overall tone of the series. Just imagine a different trajectory where Raj Koothrappali (played by Kunal Nayyar) found true love, or Penny (portrayed by Kaley Cuoco) pursued a career in fashion design rather than selling pharmaceuticals. These whimsical possibilities were abandoned, shaping the series into what ultimately aired.

Numerous storylines were hinted at but never fully developed, leaving fans to speculate on what might have been. For instance, the potential of Penny’s sister making an appearance or delving into Sheldon Cooper’s (Jim Parsons) fears remained unexplored. Here, we revisit 14 storylines that “The Big Bang Theory” left by the wayside during its 12-season journey.

The series did provide some closure regarding Bernadette (Melissa Rauch) and Howard’s (Simon Helberg) children, and a flash-forward in “Young Sheldon” reveals that Amy (Mayim Bialik) and Sheldon eventually have children. However, the fate of Leonard (Johnny Galecki) and Penny’s unborn baby remains a mystery.

While fans definitely get some resolution on Bernadette (Melissa Rauch) and Howard’s (Simon Helberg) two children — and learn thanks to a flash-forward on “Young Sheldon” that Amy (Mayim Bialik) and Sheldon will end up with a couple of kids of their own  — they never find out how Leonard (Johnny Galecki) and Penny’s unborn baby turns out. 

This is somewhat understandable, since Penny figures out she’s pregnant during the series finale, “The Stockholm Syndrome.” But even the “Young Sheldon” series finale — which so eloquently updates us on the lives of the other characters — merely suggests that Penny, Leonard, Amy, and Sheldon are still close friends, with Penny babysitting for Sheldon and Amy’s daughter regularly. 

That leaves the possibility that Penny miscarried, or that she experienced a false positive on her pregnancy test. Fans don’t learn what happened either way, leaving a frustrating blank in the narrative. Perhaps a solution will be offered up by the sitcom’s upcoming sequel series, “Stuart Fails to Save The Universe.”

“The Big Bang Theory” wouldn’t feel right without Raj’s long-lived quest for romance. Over the course of the show he dates a number of girls in spite of his shyness and inability to speak to women. Eventually, he lets his more traditional folks pick a lady for him, but his arranged liaison also goes bust. 

Raj ultimately ends the show as a single but proud dog parent to his beloved yorkie Cinnamon, and seems somewhat content with the way things are going for him. But it’s clear from his behavior in “The Stockholm Syndrome”  — specifically toward an uninterested Sarah Michelle Gellar, who comes to the Nobel Prize ceremony as Raj’s plus one –  that he’s still hoping to find the right person.

It’s possible that producers left that loose end unresolved so that Raj’s love life could become a major plot point in a future spin-off, or perhaps it felt more realistic for at least one member of the gang to stay single. When all is said and done, though, it’s definitely a plotline that was left open-ended. 

Remember when Sheldon revealed his fear of public speaking in Season 3’s “The Pants Alternative”? That was definitely a character trait that evaporated with time — and without any sign that Sheldon’s phobia had been resolved. In “The Pants Alternative,” Sheldon’s terror ends in embarrassment when Penny tries to cool his anxiety at an awards ceremony. She gives him multiple alcoholic drinks, and by the time it’s his turn to speak, he’s thoroughly drunk. The night ends with a singalong and Sheldon losing his trousers, as he ultimately moons the entire crowd. Sheldon wakes up the next morning — confused and in his underwear – as the incident goes viral.

Sheldon is required to speak in front of much larger audiences later in the series — specifically, during his joint Nobel Prize acceptance speech with Amy  — but he doesn’t waver once when that happens. Instead, he manages to be both eloquent and reflective. Since what happens in “The Pants Alternative” doesn’t ease his anxiety, this improvement in his psychological state remains a mystery.

It’s arguably the biggest mystery “The Big Bang Theory” ever gave birth to: what on earth is Penny’s last name? The show never reveals it, and when she marries Leonard she takes on his surname. Fans have argued for years over what it might be. They seemed to have an answer when they saw a package bearing the name “Penny Teller” hidden in the foreground of Season 2’s “The Work Song Nanocluster,” but this was later revealed to be non-canonical by executive producer Steve Molaro, who told TVLine, “We didn’t sanction it, we didn’t write it, and we didn’t intentionally put it there.”

While Penny may have had a maiden name at one point during the show’s production, with monikers like Barrington and London suggested at various times, it was never used onscreen and the sitcom’s producers were too worried about riling up the show’s fanbase to pick one later. “We got nervous and superstitious about giving her one. It will always be Hofstadter,” Molaro said during a 2019 WonderCon panel (via Bustle). But fans continue to wonder what it might have been.

Remember how carefully the show treated Howard’s evolving bond with his father-in-law, Mike (Casey Sander)? It wasn’t all sunshine and roses between them at first. Mike can’t stand Howard the first time he meets him and even blatantly suggest that if Howard dies during his space mission, Bernadette will get over it. But things change over the years: During “The Thanksgiving Decoupling” in particular, they find common ground over beer and figure out that they might like each other. Mike steps into the space vacated by Howard’s absent dad, and there’s a real promise between them that Howard might have an older guy to lean on when times get tough.

That’s why it’s so disappointing that Mike and Howard’s friendship doesn’t really evolve after this episode. We see Mike a few more times on “Big Bang,” specifically when he helps Howard and Bernadette with a home repair and grills Howard about his lack of grandchildren, but nothing is built upon and no further steps are taken. It’s a real shame, because there was so much more there to be explored.

In “The Tenure Turbulence,” the never-seen but notoriously ancient Dr. Tupperman dies. Raj, Sheldon, and Leonard enter into fierce competition for the guaranteed position he left behind, with Barry Kripke acting as a spoiler candidate. Only one of them will get the vaunted promotion, and they all find themselves doing embarassing, shameless things to get it. The episode ends with the news that Raj, Leonard, and Sheldon are all on the final list for the role. But there’s one big problem with this — we never find out who won tenure. 

it’s the kind of plotline that should have carried Season 6, so it’s incredibly frustrating that it never comes up again for the rest of the show’s run. Making one of the gang a tenured professor would have thrown quite the wrench into their social lives and their workspace camaraderie. Why introduce such an interesting notion, only to immediately abandon it? 

Did you know Penny had a sister at one point? While Lisa is mentioned multiple times during “The Big Bang Theory,” she never shows up to engage with Penny in the flesh. And yet Lisa clearly lingers in her sister’s mind, judging from the many stories Penny tells about her. To wit: Lisa shot her husband, and she was pregnant with her son when she got married. Penny finds these factoids bemusing, though she’s close enough to her nephew to buy him a comic book during Season 2.

In spite of that implied fondness, Lisa is nowhere to be seen during Penny’s wedding in “The Conjugal Conjecture,” and her absence doesn’t even merit a throwaway explanation. It’s as if she’s been erased from the narrative — Chuck Cunningham-style — along with her husband and son. Perhaps the drama that her brother Randall (Jack McBrayer) brings to the ceremony is too much for her family to deal with and Lisa simply slips their minds. But until we learn where she’s gone — or if she even exists anymore — her status remains a mystery.

Alex Jensen (Margo Harshman) proves to be a truly bewildering presence on “The Big Bang Theory.” Introduced as Sheldon’s assistant, she is mainly seen doing chores for Sheldon while coping with his friends. She has a brief flirtation with Leonard during Season 6 that makes Penny jealous, but this leads nowhere. 

Mainly, she shows up to be competent where Sheldon is incompetent, either socially or with Amy. She also frequently irritates him by doing things like wearing corduroy pants. This culminates in a sexual harassment complaint when Alex misunderstands Sheldon’s kvetching about women in science. He’s sentenced to a seminar, which he then forces Alex to attend in his stead. She makes one more appearance before disappearing completely, her relationship with Sheldon firmly mothballed. The show never tells us where she’s gone.

“Big Bang” struggled for a long time to create female characters that were compelling and interesting. In the case of Amy, Bernadette and Penny, and even Leslie Winkle (Sara Gilbert), they won. In the case of Alex, they sadly lost, and her presence on the show will forever remain an unresolved plot issue that could have become something wonderful. One can only imagine her becoming Sheldon’s superior, or making friends with Amy, but those ideas were never explored.

Everyone knows that Penny admitted to quitting community college during Season 2 of “The Big Bang Theory,” but during Season 6 she rejoins the halls of academia. She’s initially proud of herself for taking a history class and is determined to broaden herself intellectually in “The Extract Obliteration.” That becomes quite difficult when Leonard commandeers her first paper and decides to rewrite it so she’ll get an A. Penny tells him that she has to sink or swim on her own merits — and wins a B- as a reward when she turns in her original paper.

Did Penny finish her history course? Did she take on more classes? Did she even graduate? She may have rejected the notion of signing up for a degree program when Leonard brings it up, but would her success change her mind? Audiences never find out, which adds another frustrating layer of inertia to her character development. 

Several “TBBT” episodes involved Howard’s relationship with his absent father, who abandoned him when he was a child. In an especially poignant outing from Season 6, called “The Closet Reconfiguration,” Howard finds an old letter his dad sent him when he was younger and burns it in anger. Only Sheldon really knows what was in the note, but his friends agree to make up stories about the missive, with one of them bearing the truth about its contents.

They all give Howard kind messages except for Sheldon, who claims his dad sent him a treasure map leading to One Eyed Willie’s boon. Howard immediately knows that this is a story his friend ripped off from “The Goonies.” Fans still speculate about the content of that letter to this day, leaving behind an unfinished plotline within an incomplete story.

Later in the series, Howard discovers his dad started a second family and that he has a half-brother. Then his pop signs over his half of the family house to Howard when his mother dies. All of these new facts and near-misses seems to be building up to something — perhaps Howard encountering his father by mistake or the old man arriving on the scene to make amends. But nothing happens, and Howard never meets his father again. 

Penny just keeps losing things — like her last name, her sister, and a series of businesses. In a surprisingly ambitious attempt at being her own boss, Penny launches a hair accessory brand called Penny Blossoms during “The Work Song Nanocluster.” After getting a positive response from her friends, Penny asks Sheldon for business tips, and he suggests she open up a website.

It’s easy enough to keep up with the volume of orders she attracts at first, but Penny soon finds herself underwater when she gets a request for a thousand Penny Blossoms to be sent out within 24 hours. Since Leonard built her website and added a one-day rush order button without telling her, the whole gang is roped into helping Penny get the order out on time. They barely make the deadline only to receive another thousand-item request, and the boys promptly abandon Penny to a second evening alone with her hot glue gun.

Viewers can infer from this disastrous incident that Penny Blossoms went the way of the dodo because Penny couldn’t keep up with the chain of supply and demand. But that’s never stated out loud, and the website’s still open when the episode ends. So why did she stop selling her Blossoms?

Starting up her Penny Blossoms business wasn’t the only time Penny decided to switch careers. In “The Bus Pants Utilization,” she tells Sheldon about her idea for a phone app that combs the internet for websites selling certain types of shoes. Sheldon thinks it’s a dumb idea but helps Penny put together the interface anyway, brooding all the while over a fight with Leonard. While Penny and Sheldon repeatedly complain about their boredom as they go about the task, the show never gets around to telling us if the site launched.

The subject of Penny’s professional life has always been a fraught topic for “The Big Bang Theory” fans, which is why it’s annoying to see her have a good idea that gets tossed aside. Running the business could have brought her closer to Sheldon, mining comedy gold from their frenemy connection. Quitting the acting game may have been the worst choice Penny ever made, but the sitcom’s writers dropping this plum idea is a close second.

During their varied guest appearences, “The Big Bang Theory” made a big deal about how Mary Cooper (Laurie Metcalf) and Beverly Hofstadter (Christine Baranski) were single and on the prowl for new men. Mary confesses to dating at least two other guys during her time on the program, even expressing romantic interest in Leonard’s father Alfred (Judd Hirsch) when they share a cab together. Beverly, meanwhile, has a flirtation with an admiring Barry Kripke (John Ross Bowie). Neither woman ends up finding a permanent partner, a persistent plot hole that leaves audiences yearning for closure.

It’s especially tough in Mary’s case, because we’ve watched her grow, change, and suffer as a result of her widowhood on “Young Sheldon.” If anyone deserves a fresh start it’s her, but “Big Bang” denies her that chance. Perhaps one day a flash-forward in “Georgie and Mandy’s First Marriage” will give her the happy ending she’s earned.

Howard unexpectedly gains a half-brother, an oceanographer named Josh (Matt Bennett), in “The Fortification Implementation.” The twosome meet for the first time when Josh arrives at Howard’s house in the wake of his mother’s death to explain that his father got a letter regarding the title transfer of the domicile into Howard’s name. It once belonged to Howard’s parents, and his dad is relinquishing his stake in it.

Josh thinks it’s time they met, but Howard freaks out and runs off, leaving Bernadette to entertain Josh. Howard is furious about Josh’s existence, but before he can throw his brother out, Josh expresses his admiration of Howard’s accomplishments and they finally begin to bond after that.

But Josh is never seen again, and doesn’t even send a postcard when his niece and nephew are born. As with Penny and Lisa and Howard and Mike, a potential family connection is ignored in favor of other storylines, leaving a disappointing void where brotherly love ought to exist.