If you know that your not going to upgrade your tank, I like the mbuna idea myself. I have a 6' 135 mbuna setup and it's a fascinating tank to watch.
If a bigger tank is in your future, then Id maybe do a smaller parachromis(freddy, loisellei) or vieja. Zonatus are awesome too. Ive never kept geos, but they are very cool fish that could probably live in your tank for life.