The more I think about it, the more I don't want Doom to make an appearance just yet. Annihilus. Now that's the dude that we need to see on the big screen!!! The CGI and/or SFX departments would have a field day creating him. And he's a character that could take on both the Avengers and the FF easily. And since aliens and otherworld realms are nothing new to viewing audiences, introducing one that comes to Earth via a breach from the Negative Zone won't require much explaining.