Well, it would be interesting. But it could be dangerous for both films.
The first time a Batman - Superman - Wonder Woman live movie? Like Kevin Smith stated, the -script could be mediocre and this will still be a $500 MM movie. Outstanding -script with the joining of the Argo writer, and the revenue target goes much higher.
plus remember RDJ only sign for Avengers 2 & 3 and i think he said sometime ago that he doesn't want to do only superheroes movies when his in his 50's. for Marvel to bring out iron man 4 out it would mean a new contract has to be sign with RDJ and more money to be giving him when that money can be invested else were.