I remember hearing some people saying that they would like Kinect to be implemented in Forge. I'm not much of a Forger, and I don't have kinect, but if they do this and it leads to better maps, I'd be a happy player!
I agree with having a blank canvas, maybe something similar to Halo 3's sandbox with the three different layers. Another thing I would like is more in-depth color changing. The red/blue team stuff in Halo Reach was nice, but I think they can implement more to make stuff even more customizable.
And I know this is crazy and far fetched, but I'd love to be able to change the whether. A dark, New Mombasa esque place with rain...yes please!