There's no other choice, it should have been Warlord of Mars
. Ditch "John Carter" entirely, it's a boring, vanilla name and nobody knows the guy. The only resonance it has is that it sounds like the character on a soap or maybe a dimly-remembered doctor show....John Carter, MD, was that it...?
But a movie about a "warlord" on "Mars" is evocative and intriguing, and it summarizes the appeal of the movie - fighting, power and exotic locale. It can't be mistaken for anything else. The other important element is Dejah Thoris of course, but she can be prominently displayed in the ads, so there you got the whole package: "fighting for power and busty babe on exotic world."
Then in the next movie, once everyone know who the fuck John Carter is, they can have his name in the title. That's assuming they ever get a next movie.