I'd say you're pretty much at your limit, but every tank is different. If everything is getting along, there are loads of hiding spaces and your NO3 and PO4 stay unreadable, you might get away with adding another 1-2 small fish. Think goby, blenny, that sort of thing. Clown gobies in particular offer HUGE personality in a tiny 1" package IME.
If you're looking for visual variety, don't underestimate inverts -- shrimp, crabs, larger hermits, urchins, clams, a type of coral you've never kept before -- any of these could give your tank that 'special something' without taxing the bioload as much as another fish might.