Well you certainly have a blank slate to work with. First what you should not do. Most cichlids are out and any that will work will be very limiting. Next, larger livebearers like mollys or swords should be a no go. If you want to add a pleco, thean you need to stick with a dwarf. A common pleco will be longer than the tank. Sharks and most loaches, less Khuli will also be too large.
Here is what you can do, a small top dweller like female bettas-2-3 or a pair of pigmy gouramis would be nice. A dwarf gourami would work as well, but I would limit it to a honey or falme. For the middle, harlequin rasboras or celestial danios would be perfect. You might limit the shoal to 6-8. Neons or cardinals would work, but they really have more demanding water parameters. On the bottom, a shoal of corys, not the normal type something different. You could also go with shrimp if you're using plastic plants. If you would like a bit of odd color and movement, add 3-5 male Endlers.