Swordtails come in many different varieties and have a caudal fin extension.
If you can get your hands on some plume tail platys that would be cool.
You can also get lyretail swordtails. They have really long fins. You could get some dwarf cichlids like a cockatoo cichlid. Mollies are interesting. So are guppies! There are alot of cichlids that look quite exotic.(serpae tetras are cool!)
If you do get livebearers like guppies, mollies, platys, or swordtails, be prepared for los and los of babies! They breed ever month or so. make sure you have somewhere for the fry or else they will get eaten.
Hope This Helps!