I say no to Whedon going solo in any role. Maybe he can help write, but nothing more. I don't think anybody THAT closely related to the comics should be involved to that extent, or else the movie will likely turn out too comical.
I had a thought a couple days ago that I wanted to run by you guys. I haven't seen a whole lot of James Cameron films, but I've loved what I've seen and he has one advantage over Bryan Singer that was glaringly obvious to me when I was watching X2 a couple days ago: he knows how to use music in movies. As much as I loved X2, I thought there were parts where the music was very poorly done, usually overly done, and actually took away from certain scenes. Cameron knows how to use music, what music fits the tone of the scene, and when it's best to use NO music. I think there were times in X2 when music was out of place - most obvious, to me, being when Bobby was staring up at his family as though he was saying a final goodbye. I thought silence would've been much better there, or at least something softer than they did. That's what made me think of Cameron in the first place: his use of silence to enhance a scene.
Obviously, music isn't the most important part of directing, and I haven't seen any of his action movies, so that's why I thought I'd put the idea out for you guys to either support or shoot down.