Not a bad idea. I've played the game numerous times as Sephiroth as the main character, sort of gets annoying seeing him say Cloud's lines. Anyway, I'm not too sure of exactly what to have him say but having him arrogant or very sure of himself may help (obviously lol). Having Tifa dislike him but forced to work with him or something.
I can't remember the source where I found this bit of information but I remember reading that originally, Square had plans of having Aeris and Sephiroth like each other. Since they changed plans, they only gave her a similar hairstyle(the bangs and long hair). Could be false though so don't take my word on that.