Apologies, anon, I’m not taking fic requests currently (I thought about opening up for requests briefly during Sarumi Fest but then second short fic turned into long fic and I ended up focusing all my time on finishing that). Will just a normal reply do?
Anyway, that would be an interesting scenario – still have to age up Munakata a bit like in the “what if Munakata adopted baby!Saru” situation, since they’re still too close in age otherwise. The thing that would be notable there is Fushimi’s still got all his issues and he’s only just started to open up to Yata, and then he ends up somehow adopted by this whole new family. I think Fushimi would probably be really withdrawn and prickly at first, like totally shrugging off all Munakata’s attempts to be kind to him and probably trying to ignore him a lot and meanwhile Munakata’s totally unaffected and just lets it roll off of him. Also I see Fushimi complaining a lot to Yata about the whole thing, like how his “new dad” is so totally annoying and keeps wanting to bond and lots of Fushimi assuming that Munakata’s just trying to make a show of caring or that he’s got some kind of angle because even with Yata showing Fushimi that someone can care about him I think Fushimi would still be super-wary of adult authority figures, especially one working in a father capacity. For his part I think Munakata would figure out pretty quick what he’s gotten into and would end up treating Fushimi somewhat similar to how he does in canon, not rising to Fushimi’s bait and continuing to be kind to him and offer advice even if he knows Fushimi won’t listen to it. He’d also probably notice the way Fushimi’s becoming attached to Yata and would be pleased about it (since Fushimi having at least someone who he cares about and who cares about him in return is a good thing) while also trying to mitigate some of the eventual negative effects of that friendship, namely the way Fushimi ends up tying large amounts of his emotional health and self esteem to Yata. I think it would take a while but I could see Fushimi slowly warming up to Munakata and ending up with at least two attachments to the world then, Munakata and Yata.
(Then if we assume Munakata’s still Blue King but maybe he hasn’t mentioned anything to Fushimi and has been hiding it this whole time things start getting messy once Homra comes into the picture. I think Munakata would probably try to stop Fushimi from the attempted attack on jungle if he knew about it – since presumably Munakata knows exactly who’s behind jungle and that Fushimi and Yata have no chance at winning – but maybe he’s busy elsewhere and they still end up in trouble, and this time Mikoto and Munakata show up to save the two of them. Fushimi’s all ‘wtf my dad has superpowers’ while Yata’s totally charmed by Mikoto and they end up with a scenario where Yata wants to join Homra but Fushimi is leaning towards Scepter 4 and is all torn because he cares about Yata and Munakata and he doesn’t know what to pick. And then he possibly ends up choosing Scepter 4 but breaking it off with Yata in terrible painful ways because he’s still got underlying issues bubbling around and he can’t handle this feeling that he’s still having to lose something precious to him. So he joins Scepter 4 and is a loyal clansman, and Munakata’s pleased to have him but somewhat less happy about how Fushimi is still pulling away from everyone else in Scepter 4 and maybe clinging to Munakata even harder because he doesn’t want to lose his other precious person either the way he thinks he’s lost Yata.)