I like the grass pickerel idea. I think lake/yellow perch( Perca flavescens) would contrast nicely with the pumpkinseed you have. The different body shape and black vertical bars along with the broze coloration make them a great looking species.
Could even set up a 10g for some fat-head minnows or rosy reds to breed and add when needed give you a nice school fish and a constant food source to boot for them