Anything at all
try schools, those look cool in a hex
BIG SCHOOL OF KILLIFISH might cost alot I'd say pearl danio school with panda cories at 70 degrees and maybe shrimp
Livebearers, there are some pretty interesting choices. You could watch them breed.
It depends how you see the tank, with a shape like that I could see a very well planted aquarium with a beautiful centerpiece plant like some water wysteria or a water fern, with a planted tank like that I personally would do a small schooling fish that would provide color and movement to the aquarium such as Ember tetras, in a twenty gallon you could happily stock atleast 18 of them if you had the proper filtration setup and a densely planted aquarium, inversely you could also easily do an extremely simple sand bottomed tank with a plethora of shells and setup a nice community of small shell dwelling cichlids (I assume multies would thrive given that the floor area was sufficient to accomidate a number of snail shells for them to claim). Hope those suggestions help, I would stay away from bottom dwellers as the footprint on the tank would likely be insufficient for them to thrive.
I like where you're going, but a Hex is usually a column. So a 20 gallon column should be treated like a 10 gallon.
In my 20 gallon hex I currently have a school of leopard danios(most disagree here because of their activity but I feel they have enough room at current size, we'll see I'd suggest smaller schooling fish) I have 2 dwarf gourami males (I suggest just one but great fish) a school of cories, and some nerite snails, sand bottom lots of decor slowly adding live plants. I love my tank and feel like hex' get so much flac for their dimensions but I feel all my fish love to swim vertically as well as horizontally (especially cories)