livebearers montezumae, tail longer than you can imagine, more attitude than a midas. velifera giant dorsal fin.
cyprinids red tail shark, bala shark can't go in a 48" tank (personal experience), but maybe if its a long 75. Tinfoil barb, cigar shark, rainbows
Crencichla (Dwarf pike cichlids)
Snakeskin, opaline or dwarf giant gourami
Synodontus, doradid, or one of the giant cory catfish.
If you don't want plants, silver dollars, hatchet fish
senegal bichir
African butterfly fish, elephant nose
There are an endless number of ways you can take a 75g geo tank.