It's the only possible one unless Lucas re-numbers the current movies.
It's not impossible, Artoo. He kind of did it when he went back to Star Wars and changed it to Episode 4: ANH. I say kind of, because he only had to change 1 movie. This would be 6! movies to redo the titles... actually he even went back and redid ANH, ESB and RotJ for the SpecialEds to make them look better.
I'd kind of like to see more of what happened between RotS and ANH... maybe like Shadows of the Empire, too: The Search for Han with Bounty Hunters. But I kind of like the idea of sticking to Anakin's fall and redemption.