oh man! i'd go crazy with a 90gallon tank.
i'd wana stuff every little fish i could in there.. yep.
me + 90gallon = dangerous.
actually i'd probably just put my goldfish in it.. and make the 55gallon tropical. haha
but yeah its really cool when people get big tanks, cuz they have so many options!
if i were planning a tropical 90 gallon, i'd probably start with angelfish, or gouramis, and cory cats, those are my three favorite fish that i dont have at the moment.
another really cool fish would be the common pleco (i have one looking for a home), but you would want to have another larger home for it once it outgrew the tank. and you would want to keep an eye on it because they like to eat other stuff besides algea as they get bigger (including slime coats!) HOWEVER, the other varieties of pleco are usually much smaller, and VERY! cute! there's another thread around here soemwhere with pictures of baby bristlenose plecos, they're precious!
although it would be cool to have a tank that big filled with small tetras and community fish, i'd go with fish like angelfish and whatnot, that get too big for the "average" aquarium, and would really appreciate the space.. save then neon tetras for the people with 10 gallon tanks.
i know you seem to be more interested in more exotic types of fish. but i figured i'd put in my $0.02 anyway 8)