Oh, I love angst. MRM just is special because their canon is atually happy for once, so I'm like ... great job guys, let's keep it going <3
I do have the occasional angst craving that I put into "not hc, just a tangent, a possibility, an au if you will" territory & I'm aware for most ppl that might not be a difference lmao

My angsty fav is probably Reigen simply being reasonable. The bittersweetness of them both knowing their feelings are mutual, but Reigen not letting it happen, ever. "you have a teenage crush, you're too young to know" to "I'm too old for you, this is inappropriate" to "we might be both adults now, but I'm not the right choice" while Mob can't make sense of how Reigen can so stubbornly believe that, when he's been his choice since so damn long. When he knows that Reigen feels the same.
Plus, I think they'd never lose contact. Reigen would take Mob's kids for weekends & treat them like his own, hurting that they aren't & at the same time happy because they make him feel like he made the right decision - Mob makes such a great husband & dad & if Reigen hadn't denied him for so long, he'd never become either. Reigen doesn't even allow himself the thought that a different life could've possibly made Mob just as happy. Maybe happier even, who knows.
Mob still frequently wonders. He's dedicated to his family but whenever they're together he wonders how it would be right now if Reigen had at some point accepted his feelings. (They're muted now, but never fully gone...)
Yeah I have a hate-love with this. Makes me wanna rip my hair out thinking about but it also just feels so possible.

A while ago I rambled together some angsty "Reigen refusing Mob's advances leads to horrible things for Mob" thread, that can also be nice in an awful way ... & has angsty hurt/comfort potential for afterwards.
Hurt/comfort in general is a big fav but most times I'm like. Damn, Mob has been through enough in that one year, he deserves a forever break from trauma please, thanks.
Love exploring Mogami arc aftermath (or during) possibilities though... The angst & hurt is already canon, just gotta expand it. Sometimes I find it slightly disappointing that we didn't see trauma aftermath from that time - but also, PTSD usually sets in about half a year after the traumatic event, which would fall to after confession arc, into the timeskip. Hmmm~
I'd love seeing Mob having milk poured on him (by accident, most likely) and getting a panic attack for Reigen to deal with...