Something interesting would be "time" as a smaller theme. Just imagine a game where you actually see the impacts of your actions by skipping some years into the future. Or you could be a time traveler. Traveling through time and space to help improve earth. There could be so many possibilities with that theme.
Revenge of the Earth. The earth isn't concerned with us humans who are trying to destroy it...no, it's actually pretty pissed. Ideas could be plants or animals vs humans, environment as enemy (too harsh to go out during the day, too little water) or maybe you play nature trying to beat back the consuming hordes of people.