I agree. I was just thinking about Doomsday because of the Hulk. Brainiac and/or Metallo would be cool.
I think he might be. We'll see. If there even is a sequel. As for the box office. It'll help. If people see Superman getting punched through buildings and him doing the same to a villain in the trailer, people will see it. Of course the fight would have to be good too. If, like you said, Singer doesnt pull it off, it'll probably attract people, but it won't be good in the long run. I think it would be difficult to have Superman fighting someone like Brainiac and screw it up.
A fight won't be enough for some of the fans though. They want more changes. Although I still think even SR haters will see it....at least some will.
Anyway, I'd like to see the movie start with Superman doing his everyday job, saving people from various disasters, then have him fight a smaller villain like Metallo, then have him save the city from something Brainiac's doing or whatever and then have a big fight with Brainiac in the end.