barret - michael clarke duncan (armageddon) or wesley snipes
cloud - hmmm...keanu reeves would probably the best bet but he'd look weird with blonde hair
tifa - dunno really
aeris - liv tyler plays these sorta roles well just look at her in LOTR
vincent - tom cruise is way too short for this role and dont think he would suit it, if brandon lee was still alive he'd be perfect or maybe johnny depp but....
sephiroth - johnny depp would be better suited to play sephiroth imo
cid - ive always imagined cid as a sorta bruce willis looking guy
cait sith and yuffie - lets just quietly kill these two characthers at the beggining in fact lets kill em before the film is set so we dont have to see em