>>Teen Titans
I've been saying for years, even before Man Of Steel, that a DC film universe should start with Teen Titans. You can have the sidekicks of already established heroes forming a team or being an already formed Teen Titans. It is a good way to start with the older heroes already in their stride. Way better than whatever Gunn's Superman did.
>>Swamp Thing
>>The Authority
These are the films Gunn should have been working on instead of Superman and the mainline DC Universe.