I know exactly what you mean about tetras - mine were such a disappointment after I got blue rams.
I got a couple of veilfin angels and they are brilliant to watch although your set up might be a bit small.
I had a paradise fish until recently and he was good to watch - he thought he was jaws. he slowly swam everywhere in the tank - almost like patrolling the waters.
My dwarf gourami's are coulourful and provide a good contrast to the rams - as in they swim mid to higher waters - killifish are interesting but again are primarily bottom dwellers.
I would go for some blood red platties, or maybe five banded barbs, a panchax (stays on the surface), and a feature fish - either a paradise or a dwarf gourami.
As much as some peoples cory's are fun I haven't seen mine.
