Area of game: Forge
It would be great if we could have some kind of large platform or floor to use in Sandbox for the "skybox" or third level of the map. Sometimes you want a really big area you can walk or drive on, and using the various assortments of walls, double walls, etc, even with the increased object limits (which, by the way, is FANTASTIC) just doesn't provide the kind of surface area Im looking for. Something like the "grid" in Reach forge would be really nice.
Additionally, although I do like the "orbiting" object manipulation controls and I appreciate how it can be useful, there are times when I feel like it gets in the way. (in the original halo 3 forge, when you manipulated an object, the player remained stationary while the object moved relative to the player. In Reach, an "orbiting" system was used, where the object being manipulated remained stationary, and the player moved relative to it. For most situations, this is fine, but occasionally being able to "push" an object where you want it to go can be useful. I would not want to change the control system altogether, but if it isn't terribly difficult, I wouldn't mind being able to toggle between the two.
Another thing I would like to see in Halo3 forge is the ability to change the default physics type of new objects that are created like you could in Reach. As of now, you must manually change the physics of each object you create. If you are building something with 50 or so pieces, and all of them must be fixed or phased, that means you must change the physics of 50+ items. It would be nice to have a setting which would set the default physics of any new objects (while leaving existing objects unaltered) to save time.
It would also be nice if we could increase the maximum number of machine gun turrets on Sandtrap to at least 8, possibly even 16. The elephant has untapped potential as an unstoppable battle-wagon with VIP game variants.
Another idea would be to add the ability to "parent" or "anchor" one forgeable object to another. I'm sure we all remember the "armored warthog" idea where you place barricades on a warthog and it becomes mostly bullet proof, but eventually they fall off. By setting an objects physics to "fixed" and "anchoring" it to another object, you could have the second object remain stationary relative to the first. This also has applications with the elephant as well. Additionally, it could make forging a lot easier in some cases by allowing players to make large "assemblies" of multiple objects which can be moved as one later on.
Overall I LOVE what has been done with halo 3 so far!