JUMPS HERE FINALLY christ sorry this took me so long
anyway, I was thinking for Arthur, maybe you could take the Argent route for that? Like, ancient family of hunters, Arthur is the new head of the family and the previous head (aka Uther) was super ALL VAMPIRES ARE EVIL (aka basically his approach to magic in canon, maybe a vampire killed Arthur's mom??) and Arthur is slowly trying to turn the hunting code to be a little less nous chassons ceux qui nous chassent and more, well, the new Argent code. Spoilers: Arthur is Allison. that would give the parallel for canon things too, with the whole "but magic will be legal one day Arthur will see!!1" lmao welp. and it would also make for nice reactions from other hunters or vampires, recognizing the Pendragon name and instantly going "shit that's the house of hunters who kill all vampires on sight" and Arthur's all "WELL ACTUALLY...". and then Merlin sits in the background and wonders why this all feels so familiar.
also if Agron and Nasir are part of the "FREE THE PREYS" initiative, then I want shenanigans with Gwaine who is part of the same movement.
obviously Snow and Stiles shenanigans, MOM!!!1 wait you're a vampire now. and Porthos and Stiles because I'm a loser like that, I was thinking it might be cool if you keep for him the whole people are not cargo, mate (wrong quote from the wrong movie but shhh so sue me) thing, like obviously he's all for preys but maybe against some humans being made preys against their will? or vampires abusing their status or something? anyway, it'd play well with Stiles who is having a crisis of "wait so Not All Vampires". also Arthur and Stiles because hunters!!
idk how yours would mesh with Bucky since his background story doesn't really touch a whole lot of yours here, but did you want Korra's Red Lotus to be a thing of their own or have some relation to the scientist (Zola/Moral who has been merged into one person lmao) who turned both Art and Bucky? or maybe they're like the opposite movement if they were trying to cure Korra instead of turning her... in any case, once again there's potential for things there. also give him all your hunters, he can smile while sweating nervously what do you mean he gives weird vibes he's a totally normal bodyguard no shenanigans afoot here.
and finally, Constance and Porthos for sure since she was staying with Aramis for a while there, and maybe Arthur? Let her lie to you, Arthur, continue the tradition that all your friends lie to you at some point. also maybe Isabela, if for nothing else at least for post-event things, as in this AU Constance runs away from her husband and that could, if not really make Isabela open up about her past because lbr this is Isabela, at least in her mind give them a point of connection or something.
... jesus christ why am I allowed to write anything ever this is too tl;dr already wow I'm so sorry