Dive into your creative stream
FMA Modern AUs are so fun to me because there's just something wonderful imagining Ed having access to the internet. What do you mean he can have a mini computer in his pocket now. He's gotten too powerful.
Same Age AUs are so fascinating to me, and of the ones I've seen I really like them. In particular if we have Reigen and Shigeo both as middle schoolers because instead of an older mentor figure Shigeo is ending up with another kid. He gets to have a friend his age who gets to drag him around to do stuff and it's nice, really nice to have a friend he can relate to. Meanwhile Reigen has soooo many ✨issues✨ but hey he's great at bullshitting and is getting Shige to open up some more, while on the flip side he's also getting a friend and can go out and do more now that he has someone to do stuff with. Plus they make great business partners so this is going amazingly :D
Another version I like is if this is the result of some time travel shenanigans, mostly because I like seeing a more stark contrast between kid Reigen and adult Reigen, especially through Shigeo's eyes because... this isn't his shishou, not exactly, because kid Reigen doesn't have the years of life experience that helps his words hold more meaning, even if kid Reigen is still a great talker and bullshitter. He can't say everything right that Shige needs to hear and its a bit off putting, especially if kid Reigen has different views on being "special" and whatnot (considering... "I want to be someone"). But there's still elements there that is just so Reigen in the way he talks and the way he moves that its like. Yeah, this is a kid, but it's the same person and everyone can see just how Reigen is, this is how he started. He's not Shigeo's shishou but he's still Reigen.
I'm thinking about this idea a bit more, and I think instead of Shigeo enrolling into UA, even if the General Studies course, I'd like to see Teru take a crack at it actually. Shigeo's off in a non-heroics high school just working on bettering himself while Teru goes straight for UA's Hero course, after all he's worked with Seasoning City's totally legitimate hero and psychic Reigen Arataka.
Teru's already had his development in middle school, and now he gets to impart this wisdom upon the kids of UA, he's an amazing fighter with excellent control over his powers (are they still psychic powers or a Quirk here I haven't decided yet) but most importantly he's a great rescue hero. He always makes sure to get civilians to safety and does what he can to subdue threats, he's gotten plenty of practice in after all the stuff that's gone down in Seasoning after all.
Because it's spooky month, Demons AU where Reigen is a rather low level demon who finds himself summoned by a 10 year old Shigeo who's scared of his own powers (still psychic or something else?) and ends up contracted to the kid, maybe at first annoyed a child is who he ended up forming a contract with but over time ends up really caring and protective of this child.
Maybe Shigeo just wanted someone to talk to and there really aren't many beings in this world who could understand what he's looking for or that he could talk to so demon summoning is totally the logical choice here. Shigeo could absolutely destroy Reigen is he wanted even as a kid but nah he just wanted help :')
I'd love to see more Reigen and Ritsu interactions in fics. Ritsu is such an angsty edgy 13 year old and it's hilarious how he just does not like this sweaty salt toting conman. I need to see them work on cases together, maybe Ritsu tagging along with or filling in for Shigeo, and the two have to bond and play nice to function and get things done. Even better if there's some whump involved and they come out of it not exactly being super friendly around each other but now knowing they care about each other. There's not as much bite behind Ritsu's jabs and Reigen's a bit more open to reaching out to help the kid out when needed.
Consider; Reigen needs glasses, like his eyesight wasn't that great as a kid but back then he had these big bulky nerdy glasses he absolutely refused to wear. His eyesight got worse with age and he's still in denial about it but relents eventually and starts wearing them to read at the very least.
In an ageswap AU though I think the others at Spirits & Such would notice he's having trouble seeing things not counting spirits due to the lack of powers and they get him to wear the glasses to not strain his eyes further. He thinks the glasses make him look dorky but like there's nothing wrong with that, and Tome makes sure he gets a pair that fits well. He eventually starts wearing them full time after some development and getting more comfortable with the group, I think.
Honestly love tall Shigeo he deserves to tower over short king Teru. He either finally gets to be taller than Ritsu too or Ritsu is just barely clinging on to like 1cm over him and its too close for comfort. Reigen is dying inside over his kid all too quickly growing up and having the audacity to be taller than him while he's at it. Serizawa is just happy for him growing up. Bonus points for buff Shigeo the Body Improvement Club paid off so going into like college or something he's an absolute mountain of a guy and still the nicest most polite creature on the planet. Truly embodying the spirit of the Body Improvement Club ✊✨
I really really enjoy ageswap AUs and would love to take a crack at one myself honestly. It's fun seeing different ideas and takes on things, and I like seeing kid Reigen being a little shit. So I keep thinking about how Serizawa in canon is essentially what Shigeo would be like had he not had Reigen to help him out as a kid, and so in an ageswap au I can see Shigeo ending up a hikikomori, never leaving his room/apartment for anything other than perhaps the barest necessities.
Perhaps one time on a short, midnight run to the local 24/7 convenience store to stock up on food for the next few weeks, Shigeo sees something, just close enough to notice the movement out of the corner of his eye. Some kid with a mop of blond hair turned a dull silver in the pale moonlight and luminescent street lamps lurking around alleyways armed with a bag of salt, saying something about melting evil spirits.
It's an odd sight, but it's none of his business. He wasn't expecting anyone out at this hour, which is why he went out for food at this time, so he moves to leave, just go back home and not go out for the next long, long while.
And then an actual evil spirit shows up, making Shigeo's hair stand on end from the sudden burst of spiritual energy, and the kid makes a startled noise that catches his attention. There's not a lick of psychic energy in that kid, and there's a rather nasty spirit looming over him. It's not like Shigeo can just leave him be, the table salt the kid is violently throwing isn't even doing anything. He could get hurt.
The spirit ends up exorcized, perhaps a bit more explosively than Shigeo would have liked but it's been a long time since he used his powers. Of course this catches the kid's attention, and when said kid turns around to notice this haggard looking man in pajamas at 1 AM with convenience store bags and standing near the entrance to a sketchy alleyway the kid decides that clearly the best course of action is to run directly up to him and start chattering about "Did you do that?! Are you a psychic?!" Shigeo tries making a run for it but this is a man who barely leaves his apartment and hasn't exercised since he was like 13 or 14 for PE classes so the kid catches up embarrassingly quickly.
The kid enthusiastically introduces himself as Reigen Arataka; Rising Star of the Psychic World and soon to be Greatest Psychic of the 21st Century. The kid, Reigen, goes on and on about how he was looking around for an evil spirit he's heard about from classmates at Salt Middle and from gossiping housewives he's passed by, and obviously since spirits are more active at night it's a great idea to go looking then.
And since Shigeo's a psychic, that means he has to teach this kid how to use psychic powers. Obviously.
Shigeo is not thrilled at the idea. Using his powers right then was a one time deal and he's not going to use them ever again because he'll explode and hurt everyone and-
But Reigen's not going to take no for an answer, he gives Shigeo a card for a sketchy looking business called Spirits & Such, run by one Kurata Tome. Reigen says that's where he's working after school and on weekends, and that he'll be taking Shigeo there the next day. Somehow, Reigen manages to get Shigeo's name and address from him before prancing off into the night like it's a totally normal thing for a what, 13?? year old to be doing. Shigeo goes home for the night, that was way too much interaction for him for the next decade. And it's not like that kid will actually remember this and show up at his apartment, right?
Shigeo wakes up to insistent rings of the doorbell some time in the afternoon, and he's content to just not answer but damn that is loud and persistent and just wont stop ringing so he begrudgingly cracks the door open to find Reigen in all his like 5'1" glory ready to drag Shigeo out of his apartment and to Spirits & Such, calling him Shishou all the while.
So it turns out this Kurata person recognizes his name as that kid who refused to join her Telepathy Club in Middle School, which is awkward. Shigeo's not quite sure how he got here, getting a job and a makeover even (Kurata insisted she cut his hair rather than let Reigen handle the scissors) but Reigen's persistent enough that he finds himself falling more into a routine working at S&S, and it's... it's nice, actually. He's finding his anxieties quelling and himself relaxing more, and talking with Kurata and Reigen isn't all that bad. He's getting himself back on his feet. Reigen is impressive with how he speaks with customers and a part of Shigeo wishes he could do that, wishes he could speak so clearly and understand people so easily. He's working on it, bit by bit.
And then he finds himself questioning his life choices because of a spirit haunting an all-girl's high school, shutting down a cult and gaining a sentient fart cloud as a companion, running into a TV celebrity who happens to also be an esper, and a whole lot of other things go down that leaves Shigeo's head spinning.
I've been reading a lot of MP100 x MHA fics and looking at prompts and I'm absolutely in love with the idea of Quirkless Reigen faking having a quirk, maybe even running his own (not exactly legal) "hero agency" in Seasoning City and getting the most powerful sidekicks and interns possible despite being relatively unknown as a hero (also not exactly legally one). His "agency" is more focused on just helping people with whatever issues they may have be it spiritual, Quirk-related, or otherwise really.
I just think it would be interesting if maybe, one of the kids he knows (maybe Teru, even) placed really high on the UA Sports Festival and ended up dragging along other characters to Spirits and Such because that's where he ends up interning. Others are like, surely the Hero who managed to get Hanazawa to intern at his agency is super powerful and knowledgeable right? Right??
What they end up with is a sweaty man in a cheap suit (is that supposed to be his hero costume??) claiming his Quirk to be so powerful and out of this world he can't use it for most situations lest he destroy the city block or something, who gives them much needed life counseling and lectures and takes them out to help citizens with the most mundane things under the guise of "evil spirits" or "villainous plots".
He has maybe two, three employees there, though they don't have many work hours anymore, they do pop in sometimes and help the interns learn about the job, the paperwork and attitude and helping others. And thus they learn the power of customer service that it isn't big flashy fights or being super popular that makes a hero, its just being a good person and helping others.