Welllll, short answer is "nearly everyone... except the ones that likely would get expanded on" I mean out of such a big and versatile cast, the Crazy Cat Lady is the one you choose to expand on? At least I can safely say "they have no excuse not to focus more on (x)" I guess.
For some specifics:
Ms Hoover: Already been mentioned. Bonus points if it actually divulges from Edna's. Like never actually wanting to be a teacher but pressured into it by parents because what she wanted to do wasn't "respectable" more points if it also happened to be a passion of Lisa's. That'd set in a connection for a plot.
Most of the retirement castle: For pretty simple reason, they've basically lived their entire lives. Apparently the old jewish guy used to be a director or something. But yeah, they've got like 70+ years to shove a backstory into. Jasper seems to be Abe's best friend but as far as well can tell, they never fought together in WW2 or anything so maybe it was something else that drew them to hang out. Or maybe Jasper was the brave hero that finally managed to reclaim the word "twenty" from the Kaiser I dunno.
Janey: One of a few good examples of the show coming out with something of interest almost assuredly by accident since she was only ever meant to be "that one person for Lisa to occasionally talk to". However there's been a few things between episodes and side materials like Tapped Out that seem to imply she has rather low confidence. Probably the most telling in-show was in Lisa's Substitute where she claimed to have no talents at all and in Tapped Out Marge goads Lisa into doing a recital by insisting her backing out would give Janey the boost she needs to find her own voice and possibly outperform Lisa. The outperforming bit is probably exaggerated (some of her lines particularly in the comics portray her more as a bit dim) but she is mentioned in one of the books to fit in by copying the more popular kids. So maybe her infamous flakiness is more that she doesn't feel confident enough to asset herself in the ways she wants to? It could also be that in that area, Lisa's influence on her life is actually poisonous, how
can someone feel confident when one person gets every single award thrown their way including ones that feel made up just to give her another prize? For that matter, Lisa has consistently had a hostile attitude towards anyone that threatens to even perform on her level let alone better, like with Allison and Maggie and, again comic example, even fantasized about hanging Martin over a piranha tank just because he won a spelling bee against her.
Sherri & Terri: Another accidental case, for sure but there's still stuff to consider there. Particularly how contradictory they're written. They tend to get shoved into the "alpha bitch" role but whenever they do... it's just them. And any stereotypical "popular bitch" has to have a following of more than just a borderline conjoined twin sister. Maybe sometimes they won't be alone but then they're never the ones that actually have lines then, they're just hangers-on. Is that how they may have been treated before? They do seem to
want to be amongst the cool kids so are they just imitating the same behaviours to someone lower than them on the social ladder? They do seem to act pretty strange when they're on their own. Or is it a personal dislike of Lisa? Ain't like there's no reason for them to hate her and they're oddly accepting of other kids you think an actual Libby would avoid like Allison or even Ralph, with Sherri even implying she has a crush on the latter in the simpsons game. Really the way the writers present the issue ("The other girls don't like Lisa because she's too smaaaaart :<" They like Allison just fine, even if she was a bigger geekazoid than Lisa) and a good writer would pick up on that. But then it'd probably need to look at Lisa in even the most mild and constructively critical way and I wouldn't be surprised if there was a mandate against that considering some of the messed up crap she'll never be called out for, let alone hit by a bus or drowned for.
I'm not sure about Frank Grimes himself but Jr could probably do with a bit more exposure. I mean to have a character with a connection to one of the most significant one-timers essentially be a throwaway character only seen at the tail end of one episode. Yeah, who even remembered he had a son? I guess the bit on Grimes "liking hookers" could be brought up more, like if it was some sort of comfort considering his crap life. I guess you could look into both of them using Jr?
It'd also be kinda neat to see the Shelbyville crew again. I think the only other time any of them showed up was an unknowing team up between Shelby and Bart, as their superhero personas, in which he had a villainous sister who was obsessed with squirrels. Actually, just Shelbyville in general. If it still exists that is, I think the last we heard of it was in 24 Minutes where it got nuked... I mean it did get better after the toppled sun blocker but still.
But realistically, we're probably more likely to get the story of Yeeeeeesssss guy or Lenny's third toenail than taking advantage of any of the many, many opportunities that are out there and... wait has Lenny even had a story to him? I mean there was the saga of Carl but despite getting a lot of screentime I'm having a hard time thinking of anything that Lenny had going for him aside from a mystery novelist with eye problems...
Oh, should be worth mentioning that not everything, even if there's a story to it, needs an entire episode dedicated to it right off the bat. After all, a fair few plot-relevant details such as Moe's depression or Nelson's home life issues came before the eps they were related to. Least iirc. Start small and all that.