Because of size:
aro's
clown knife
RTC or TSN
Gars
Fire Eel: because i tried a peacok eel and it died/dissapeared, so I'm nervous to try another spiney eel.
Africans: I've really wanted to keep an african tank for awhile now and have been waiting for the right time...which is hopefully this summer.
Piranha: I really like these guys, but haven't been willing to dedicate an entire tank to them yet.
Wolf Fish: they are at every LFS around me, but I don't think they work in a 55, and my empty 180G is awaiting my sunnies (once they get bigger)
Oh yeah...
Discus: they are really pretty, but their price tag and delicate nature has kept me at bay for awhile.