Does John Carter beat up the Silver Surfer in gladiator-style combat? A Planet Hulk movie would be awesome.
As far as a Black Widow movie goes, it would make far more sense to just do a damn S.H.I.E.L.D. flick and give Widow, Hawkeye and Fury plenty of development and spotlight. It could use small cameos with The Avengers and deal with what S.H.I.E.L.D. has to do to keep superhuman crime fighters in check.
Don't forget Maria Hill.