this is a rough idea i've been thinking about that i feel at this point would be best described briefly then added to by other creative members of the community. it's essentially a hypothesis on fusions' nature, and most specifically how it affects their appearance as well as behaviour. little bits are inspired by existing content including forum posts and official stuff from both OG FF and FFL.
in short, my idea is that a fusion takes on the characteristics the original counterpart dreads the most about themselves. this includes things they regret, things they are self-conscious about, and things they fear could happen.
i first started considering this when the devs confirmed that fusion Father would resemble Benedict Uno , his pre-villain self. i thought that was an interesting choice, because it gave the impression that the fusion took on that appearance almost to antagonise Father, reminding him of a time when he was cowardly and weak. taking this forward, i thought about how other fusions in lore exaggerated dreadful characteristics of the original - such as Bloo. while Bloo is quite greedy and is a bit of an egomaniac, i remember reading about how fusion Bloo was grotesquely massive in every way, with obnoxious features to go with its obnoxious personality. this stuck out to me as a way of fusions preying on a character's insecurities, and looking at other characters now i think it would be a really cool way of exploring the psychological aspect of how dangerous fusions are to their originals.
there's a couple of characters i've been thinking about that could really make this idea powerful, which i'll list as i think of them:
Fusion Mojo Jojo - Jojo the Chimpanzee
if there's one character that has the potential to bring a lot of depth to this, it's Mojo Jojo. considering his backstory, his fusion taking on the appearance of when he was first mutated would constantly remind him of the day everything went wrong for him. according to my research some of the IDW PowerPuff Girl comics develop his conflict about his origins quite a bit, and his wiki page quite accurately notes its influence by the rise of Caeser in the Planet of the Apes
other than Jojo, I've thought about Finn/Farmworld Finn, Kevin/Mutated Kevin, Grim/Grim in his prime. i think Grim/Grim in his prime could be a fun one as Grim in the show is quite mellow, showing in moments that he actually cares for Billy and Mandy somewhat. however, seeing fusion Grim's more aggressive appearance, maybe it could reflect a time when Grim was much younger and ruthless, full of anger. this version of Grim is something he may not be too fond of simply because he's matured with age.
but that's my kind of concept. i'm kinda just trying to jot everything down in order to make more sense of it after some conversation.