There are almost too many concepts to choose from without knowing what you envision,
An African rift lake tank with many colorful mbuna, and a rock pile in the center.
An Amazon river tank centered with lush plant growth (aquatic and terrestrial) , with shoals of tetras, Geos, and Angelffish
That size tank is approximately the territorial size of one semi large pair of Central American Cichlids,
Or it could be a shoal of "social" Central Americans like Thoricthys (fire mouths) or Cribrheros surrounded by schools of live bearers.
A brackish mangrove swamp theme with archers, scats, with a emerged area in the middle for mudskippers.
An Australian theme with large shoals of rainbows;
as you can see, I think geographically.
But these are my preferences, and other people will propose theirs, then again you might prefer some totally different concept