If Superman does get removed, I too wonder what might replace it in the coming years.
Six Flags has worked with Larson, so there is there possibility for one of their drop towers.
Six Flags has also worked with S&S, and the company recently announced the Spin Shot Tower. That would be a new and unique experience.
I also wonder if there is the possibility of some sort of (Intamin?) drop tower being attached to the Mr. Freeze structure. I have no idea if that would actually be feasible though.