The Simpsons has a lot of one-shot characters that are quite popular, such as Jessica Lovejoy, Allison Taylor, Alex Whitney, Colin, and every guest spot child character. Many fans are unhappy that they only got a single appearance:
- Allison Taylor in particular is a strange case, as twice during the Scully era, she did have lines, done by Pamela Hayden (who has been around for nearly every episode) but was dropped entirely afterwards and her role as the student Ms Hoover likes better had been taken by a pointless expy in Hubert Wong despite having no reason why most of his moments couldn't have gone to Allison since he was mostly only namedropped without any lines. Not only would she have rounded out the Jerkass to Nice Guy / Nice Girl ratio but her presence as the favourite of the 2nd Grade would make Lisa's plight about being unappreciated make more sense than it does in the show as it is. She shows up almost constantly in nonspeaking background roles making her current lack of significance even more glaring.
- Hank Scorpio. The man was a genuinely nice, down-to-earth... James Bond supervillain. He's every bit as awesome as he sounds, and "You Only Move Twice", the episode in which he appears, is generally regarded as the best episode of Season 8. He could have been a great recurring villain or Deal with the Devil character (he offers Homer a job in "You Only Move Twice"), especially as Mr. Burns became senile and ineffectual in the later seasons, but no luck. For the record, this guy was almost made the Big Bad in The Simpsons Movie.
- Samantha Stankey is a good example to apply to this trope. Not only could she have been the one true Love Interest for Milhouse (and end Milhouse's embarrassing attempts to win Lisa over), but she could have been a second best friend for Bart, making the three of them a trio, and by possible association, the first true friend Lisa would ever have. Sadly, Samantha's prudish father sent her to an all-girl convent school, where she is locked away from the outside world, and hasn't been seen or referenced since. On an analytical note, Samantha also could have been one of the rare glimmers of purity in a lackluster town full of jerks.
- After regaining his wealth and reconciling with Homer, Herb Powell could've been the Big Good of Springfield, given his status in contrast to Mr. Burns' Big Bad. He could've also been an excellent benefactor in helping the Simpson family in any of their recurring financial jams. However, by the time the writers decided to get Danny DeVito involved with the Simpsons again (now on the onset of the Great Recession), he was given one off-screen line confirming that he was poor again.
- The Nahasapeemapetilon Octuplets themselves could've been given individual quirks that distinguish them from each other (if they would get older and receive more screen time). However, any future incarnation would involve all eight of them working at the Kwik-E-Mart as eight mini-Apus, and being bossy and rude to any of their employees.
- Even though Ling Bouvier was able to be adopted from China by Selma, both Patty and Selma are shown more frequently than her own baby (which raises the question, who is watching their baby?). In terms of possible use, Ling could've served as a playmate for Maggie. Though later episodes have her and Maggie (and sometimes, with other kids) in the background for side jokes.
- Maude Flanders deserves a special mentioning. Given Ned Flanders' reputation as the nicest neighbor in Springfield, and Marge's constant pleading for Homer to be on good terms with him, one would think that Marge and Maude would get along better than their husbands. Unfortunately, Maude is shown as nothing more than an overprotective mother and a religious fundamentalist who is more on par with the gossipy Helen Lovejoy, and has been critical and disapproving of Marge on separate occasions (including hiring by the Yakuza to stomp out her pretzel business). By Season 11, the writers decide to kill Maude off, mainly due to the dispute with her voice actress Maggie Roswell.
- Mona Simpson. She loves Homer and regrets abandoning him, in contrast to unrepentant Eeyore-Jerkass Abe, so she could have provided a different perspective on his childhood, and her history of activism and radicalism could have made her an interesting partner or foil to Lisa.
- Ruth Powers and her daughter Laura were introduced in season 4 as the Simpsons' new neighbors. In this case:
- Laura acted as Bart's crush and could have become something of an older sister to him at the end of her first appearance, and not only could she have easily filled the role of babysitter for Bart and Lisa, but have episodes on showing Bart some of the tribulations of going through growing up into becoming a teenager through his friendship with Laura. Despite this, she completely vanished after her sole appearance in "New Kid on the Block" and didn't even appear in "Marge on the Lam" or "Strong Arms of the Ma", despite said episodes bringing her mom Ruth back.
- In the next season, Ruth had her own episode where she further bonded into good friends with Marge and went on an adventure with her. The two could have played a larger role in many Simpson's adventures, but aside from a few cameos, Ruth herself only made a single appearance a decade later.... only to be depicted as a steroid abusing ex con who badgers Marge into also using steroids. Ruth's marriage situation also seemed like a total mirror opposite to The Simpsons, in that she is only two years younger than Homer, and would have provided insight onto what the single parent life of dysfunctional families were like in The Simpsons universe, including having been married to the near foil of Homer (hard working, loves his career, but is a lazy drunk slob at home and doesn't even care about his family) and everything hard knock life on her end.
- Rita LaFleur, Homer's long-lost stepmother. She was an interesting character and added some insight to Abe's life after Mona. Despite them reconciling at the end of her episode and continuing to develop their relationship, she's never seen or mentioned again. Probably justified as she was voiced by a guest star, rather than one of the in-house cast members.
- Julia, a beauty stalker who tries to seduce Homer and, not accepting refusal, she tries to kill him. She could have been the personal Sideshow Bob of Homer.
- Or the Sideshow Bob of Marge, since she clearly intends to have her revenge on her (with or without help from the cable company).
- Other one-time characters who could serve as personal Sideshow Bobs for the family include Frank Grimes Jr. for Homer, Mrs. Cantwell for Lisa, and Jack Lassen for Bart.
- Walt Warren from Season 21's "The Bob Next Door". This man first started out as a low-level criminal, and was scheduled for early release after Mayor Quimby released minor offenders to save money, but Bob gets to go instead thanks to his latest Evil Plan to kill Bart. For the rest of the episode, Walt does everything in his power to stop Bob and save Bart, actually showing off some pretty smart feats in the process. He forms a perfect plan to escape from prison, and doesn't trust a certain waitress when she "tries to throw him and the rest of the Simpsons off the scent after falling in love with Walt's face", and would much rather continue on the proper route to save Bart. Most criminals, even minor offenders, probably wouldn't go that far to save one boy whom they've never even heard of, but knowing that Walt would shows a true hero. A true hero who never gets the thanks or proper goodbye he deserves. The Simpsons never thank him for his help in stopping Bob, and Walt himself just disappears from the rest of the story, despite that the house next to the Simpsons' is legally in Walt's name since Bob purchased it with his identity.
- On a smaller note, the episode ends with Flanders' cousin Ted moving in next door - with his two daughters Bonnie and Connie. Any stories about the Simpsons dealing with this (Homer dealing with two Flanders, Lisa having new friends to play with, Bart getting one or both of them as love interests, etc. etc.) are immediately thrown away as the credits roll.
- Some fans feel that two-time character Rachel Jordan should've married Ned instead of Edna, given their good chemistry (aside from Ned's awkwardness of picturing her as Maude and cutting her hair into a similar hairstyle as his last wife's). Others feel that Flanders could earn his happy ending with this, given that his previous two marriages have ended in deaths of both wives (with the latter being dead along with her voice actress).
- Josh Weinstein wished Frank Grimes could have survived past his debut in the DVD Commentary for the episode, owing to his complex character and contrast to Homer, as well as him being a brilliant Deconstruction of the Simpsons universe.
- Milo was originally introduced as the Ned Flanders to Comic Book Guy's Homer Simpson — someone who seemed to outdo him in all respects. He's a rather positive portrayal of a nerd, who gets along with Bart and Lisa and even has a healthy relationship with a girl. The writers apparently struggled to follow through on this idea, though; in his debut, he drives Android's Dungeon out of business, but this is never resolved as It Was All A Dream. He's only made one appearance since, not counting silent background appearances. (Admittedly, CBG is not that prominent a character, and Milo's original voice actor — Jack Black — couldn't be around all that regularly.)note
- It's a given that we can only see Bart's and Lisa's children in the (fairly rare) episodes set in the future, but with that in mind, we learn almost nothing about them, aside from "have strained relationships with their parents but love them deep down". We don't even really learn how their relationships got strained in the first place. Bart's sons weren't even given names until years later when they were made available in The Simpsons: Tapped Out.
- Many of the kids, especially the girls, get the shaft in more recent seasons, but none more so than Sherri and Terri. In the first few seasons they were clever and tricky, knowing how to play Bart like a fiddle, and could have served as rivals to him yet more lighthearted antagonists compared to Sideshow Bob or even as occasional allies as they shared some common ground and one or both are implied to have a crush on Bart. Later on, they were shifted to being associated with Lisa instead of Bart. Not that they couldn't have worked with Lisa in theory, but in practice, this completely took away any significance outside of sometimes having a few seconds to set up a "Lisa has no friends" plot, making jokes about them being twins or being subject to some incredibly over-the-top painnote .
- Jessica Lovejoy is the trouble-making daughter of Reverend Lovejoy who served as the genderswapped foil to Bart, and it was clear that there are problems within her family. She becomes conspicuously absent in later appearances, especially when the Lovejoys are actually given the spotlight (in which she just disappears completely).
- Miss Pennycandy, personal assistant to Krusty the Clown. She's the one who pushes Krusty to have dinner with Bart to make up for his previous cancellations, and is shown to have an unhappy love life and feelings for her boss. She could have served as a voice of reason to Krusty, or explored her feelings for him further, but the character was dropped.
- Janey Powell, Lisa's supposed best friend. Despite Lisa consistently being portrayed as an unpopular loner, several episodes either show or mention her hanging out with Janey. But the show barely uses her, and when it does, the scenes where they were playing together were barely plot-relevant, even in the old days. Worse still, other episodes show Janey had no qualms ignoring Lisa or making fun of her with their fellow classmates. A shame too, because what little we've seen of them playing together already gave us glimpses of a personality contrast (namely that Janey is more carefree than the responsible Lisa) which would have made episodes focused on the two all the more interesting.
- Moe's girlfriend Maya from "Eeny Teeny Maya Moe". Not only is the episode very well-liked for showing Moe in a relationship with someone who genuinely likes him, "The Wayz We Were" has them reunite after a few years and actually get engaged — with this not being reverted by the end. Exploring how Moe would go about getting married and having a girlfriend would be an interesting change for his character, but Maya is never mentioned after this, with the show still portraying Moe as depressed and lonely.
- Of all the characters exclusive to The Simpsons Movie, Colin was the one who got the shaft in the episodes that followed. Russ Cargill, Spider-Pig, and the "Boob Lady" were all lucky enough to receive cameos here and there, but Colin completely disappeared. His disappearance doesn't even make any sense, since his arc in the movie ended with him and Lisa romantically holding hands as they walked into the sunset, with no signs of an impending breakup or separation.
