I just had a very interesting thought about this. What if, for a twist, it turns out the villain in "Fury Road" turns out to be Toecutter, the bad guy from the first film (yes, I know it's the same actor, but I mean the same character)? Could you imagine the surprise of finding out that Toecutter didn't die in the third act of the first film like we thought, and went on to become this new villain in the film? It would be awesome to me if it turned out that Toecutter survived the first film and then went on to become this even more ruthless and evil bad guy, while Max went on thinking he killed him. And now fate has put Max and Toecutter back on the same collision course once more, and now Toecutter has an army to back him.
I know, it sounds way too out there, especially since Miller has reused actors for different roles in the series before, but I think this would be an awesome twist.