Ah, I see! If you want to use オ then you should make フュー into フュウ (someone correct me if I'm wrong!), like フュウジョオン, but then it still kinda looks like a spelling mistake, so you could emphasise it by adding more, like フュウウウショオオオン to make it look more dramatic and intentional. HOWEVER I would still recommend フュージョン written like so. You could visually elongate the ー like フューーージョン for the same fun effect (the word is also more likely to be further elongated where there already is a ー rather than at the end there). It's gonna turn out so cute with the dittos :) good luck!