I wouldn't do a big aggressive CA cichlid like a vieja, trimac, midas. They are on a different level of aggression than the more peaceful severums, Oscar, etc that you have, likely to dominate the tank, and the other cichlids won't be ready to deal with it. I would stick with other south American cichlids like maybe a hoplarchus, or the heckelii mentioned. A crenicichla atabapo would work too.