You have plenty of options if you like cichlids there have been some good suggestions. I think you will like the SB flowerhorn if you go with it. There are some others that might be fun as well. Some may disagree with the size fish, but what else is a forum for if everyone agrees all the time. Sajica are a nice fish that stay smaller. In fact most of the criboheros would be a good fit. Many of the acaras would be nice. Bartoni might be pushing it but that is a stunning fish. Don't underestimate the value of one male firemouth. One buffalo head cichlid would be fun. I used to keep them and they were very interesting. A regani pike could be fun in there. The big problem with them is people usually try to keep more than one.
Since you are so open minded and I don't know what you want out of the tank. Below are a few other options that would be very different experiences. How you scape it will make or break how nice some of these fish are.
If you go the leporinus route I would go with Leporinus granti as they stay smaller. Abramites would be in the same category.
Some that seem strange to me to keep alone, but fit the space requirement would be the peacock cichlids.
The arrowhead or hairy puffers would be fine in there.
If you went brackish you could have green spotted or figure 8. If you went brackish you could also do a mudskipper, but if you wanted to go that route you probably wouldn't have put this in the general freshwater forum.
Depending on your thoughts about a very flexible fish in a 12" wide a peacock eel could be fun in there. They are very interactive once acclimated. A dragon goby would be a similar situation.
One clouded archer could work in there.
African brown knife
African butterfly fish.
If you really like catfish a bumblebee cat could be interesting. Or for an active catfish hoplos or megalechis are cool.
Snakeskin gourami because they move so slow could work, but any of the other small/mediums would work like moonlight, leeri, blue, gold, opaline. If you have ever seen a huge opaline it is a much nicer fish than most people realize after looking at them at 1.5" in the store.
One everyone will make fun of is a Chinese algae eater. If you have ever seen one at 8" it is pretty impressive. They get tiny horns on them.
One cool pleco isn't something to rule out either. I like the king tigers.
A non fish option would be an African Clawed frog. They are big and fun to watch devour food.
I could jump in and recommend a larger tank and you will be happier with it, but your constraints are real and it doesn't answer the question you asked. I do agree a 40B is a better tank with the added depth. If you do get that 90 your options are very wide.