So, if you don't play Yu-gi-oh!, fallen of Albaz is a card that can use your opponents monster to fusion with himself. I think that it's a pretty cool Idea. Forcibly fusing yourself with an opponent to evolve sounds like a fun idea.
Well, here's my 2 ideas. First one is basically, you can fuse your opponent's power with yours after you've defeated them, basically taking their power. While this is cool, it aint unique.
The second one, you can fuse with your opponent after you have touched them (or in case of Alba-Lenatus, just by having them in a certain radius), the main character will transform into one of his many forms. (I aint writing all of them, look the cards up on your own) You can also use other archetype cards as "powers". Something like branded fusion making you instantly evolve, but you will lose your mind or something. (p.s also add despia in someway, it would be fun)
There are a lot of potential with using yu-gi-oh! cards as powers honestly, but as a fan of branded despia I think about this one all the time.
Off topic: Exams season is starting for me and I am completely unprepared. Also I have been replaying the south park RPGs the fast few days. Would definetly recommend.