It is left to be seen if Man of Steel is atually a first contact film. There are rumors that there were other aliens (like Martian Manhunter). Green Lantern 2 may turn out to be a soft reboot.
It was confirmed that MOS will be the first time humans discover the existence of alien life. Martian Manhunter already being on earth wouldn't count if no one knows he is an alien.
I would personally be more interested in a Green Lantern Corps movie (Hal could still be the main character) instead of GL2.