Most cichlids that stay 12" and under would be fine in a 90 gallon. Istlanum, as mentioned, would be a good choice. Motaguense, grammodes, loisellei, would be some other good options.
For non-cichlids, I would say upper jaw bichirs would be cool. Also, gar-characins like the needlenose or hujeta gars, would work.