A 90g would be enough for some of the bigger species of fish, just not the *biggest* species. Keeping with NW Cichlids, you could do severums, blue acaras, jack dempseys, just to name a few.
What that would be perfect for would be an African cichlid tank - they are stunning and due to the way that most tanks are planned out (overstocking to distribute aggression), with proper filtration, you could get more bigger fish than you could in a tank of another kind.