Its plausible, as of a few months back i was hearing rumors of a Bats/Supes cross over film being used as the 4th installment of the respective franchises that were launched w/ Batman Begins and Superman Returns.
Prior to BB getting Greenlit the "Worlds Finest" (Comic fan short hand for the team up) project was planned as a start of the two franchises. but it fell through.
Now the last I've heard is that a 'Justice League' film has been approved, which will feature Batman (Not played by Bale), so if it too features Superman its plausible that a Justice League Sequel could be 'Worlds Finest' or 'Trinity' (Wonder Woman and the other 2).
However since the \S/ used in Legend was the Logo from Returns, but the Bat symbol was not the one from Begins, it could be that either the Logo changes in Dark Knight, or it was just a 'theoretical film' used to denote that I am Legend is set in the future, in its own Universe.