In short, no one knows the answer to that question. On one hand, Robert Downey Jr. could have very well been campaigning on Mel Gibson's behalf, and on the other hand, it could all be a bunch of hooey. As of right now, the rumors indicating that Robert Downey Jr. has been campaigning to include Mel Gibson and Jodie Foster in an Avengers sequel are just that - rumors. But let's suspend disbelief for a second and explore what it would mean if the rumors are true.
First, it would mean we have a damn good set of veteran actors on board. Say what we want about Mel Gibson's personality, he's an excellent actor who would add an immense pool of talent to the Marvel universe. He's also proven that he can pull those kinds of rolls off; imagine adding an old, grizzled William Wallace to the Avengers and you can't help but be intrigued.
It would also mean that someone who made huge headlines for his anti-Semitic comments and inebriated rages (Mel, not Jodie) would have a part in a Marvel film. Who owns Marvel? Disney. Who runs Disney right now? Bob Iger, chairman and CEO of The Walt Disney Company. As it happens, Iger was born to a Jewish family. Even if Robert Downey Jr. were able to convince a casting director to take Mel on despite his questionable past, would Iger let this slip by? Doubtful.
In the end, the assumption is, it doesn't matter whether or not Robert Downey Jr. is fighting to get Mel Gibson and Jodie Foster into the Avengers sequel, we probably won't be seeing Mel grace the Avengers scene any time soon, but we can always hope (or breathe a sigh of relief?).