That's why my list of suggestions is so narrow; it's hard to find people who match both the voice and appearance of the characters (especially the original character designs).
That being said, I wonder if Juliette Binoche would make a good voice actress for Aeris. She has a waifish innocent quality about her that would complement the character. Except maybe the accent would be a problem?
Tifa has always been a problem for me; she's such a contradiction in terms of appearance and personality. Seeing Hillary Swank in Million Dollar Baby has convinced me she could do the physical part but I'm not sure she has the right voice.