A few fish that come to mind are Jack Dempseys, Oscars, Red devils & Jaguars for other cichlids however there are a number of S.A. Catfish (Shovelnose, Redtail, Armored, etc) as well as Pacu and if your state allows them, Black Piranha that can also go in with him/her. It will all depend on what size tank you call "massive". There needs to be a lot of length in a tank when dealing with territorial fishes. Outside of the S.A. fish are fish such as Clown Knifefish, Channel catfish as well as other larger growing catfish from around the world. These are just off the top of my head. I'm sure there are plenty of other fish that can go in there as well, again, all according to how much other territory is available in the tank.
Hope this helps