Perhaps adding some larger smallmouth bass would give you the diverse fishery you desire. If you have stunted bluegill and 15 acres I think there is a good chance the smallies will do well, especially if there is a deeper area and/or rock cover in your pond. The largemouth will always dominate the pond but you could end up with a nice population of smallies in the deeper water. As long as the bluegill are small the smallies should be able to feed on them, adding an additional forage fish like golden shiners would only help. You could catch 10-20 12"+ smallies in a local body of water or try to buy some 6-8" smallies in the summer. If you stock wild fish you run the risk of introducing disease so be careful and don't stock any fish with rashes or open sores.
