In theory they dont "need" a topping but people cant help doing it. A good colorado like Shunka say's is really nice, or a good side of spicy beans.
As for absolutely insane? The dreaded "nachotamal" that a place in my hometown sold. You start with a tamal in a very large styrofoam container. Cover it in tortilla chips and bathe these in spicy, runny beans. On tops of this you pour tons of homemade nacho cheese, jalapeno slices and crumbled dry white cheese. By the time your full you realize that there's a tamal somewhere down there you havent even touched yet.
My english, she's not so good... I meant to say I did it with the malice of forethought.