I agree with pretty much everything OP says. I have a few ideas of my own though.
1. This would be really complicated, but awesome. Dynamic water (IE water can come from a source and will flow realistically, and you can swim in it.) Also, boats.
2. In a video posted recently I saw something that looked like an anti-gravity field (the player drove a mongoose through it and it went up instead of down.) I don't know if this is going to be in the game, but hopefully it is. A cool feature that could be added to that is that your orientation will shift with gravity, and gravity can be set in any direction. Imagine if there was a floor, and when you went beneath it you would flip over and start walking on the underside of it. Also,a gravity sphere that pulls players toward the center, allowing for mind****ing maps.
3. Usable machinery. This is similar to OP's idea for doors or elevators, but I would add things like light bridges or force fields, or even teleporters that can switch channel or be turned on/off.
4. Ammo crates. Think of the ammo boxes from reach firefight, except you can customize them to have limited ammo, or only certain types of ammo.
5. Separate layouts for each gametype. You would build your default version, then go back in after selecting a specific gamemode and make the changes you want. This would all be saved under one map, but would allow you to make minor weapon and layout tweaks for specific gametypes.
6. In-match purchases. Think of how each kill and medal and whatnot gives you credits. These would be added to an in-game total separate from the progression system. You could then use them to buy things like power weapons or call-ins (think orbital strikes or ammo crates, similar to tribes:ascend).
7. Forgeable firefight. It would work just like regular forge except you have to place enemy spawn points as well. This would be HIGHLY difficult to add given the pathing issues that would arise, but it would be soooooo awesome.
8. Duping. I know that they let you dupe objects in game, but they should let you select multiple objects and then dupe them as a whole, single object. They might let you do this in 4, I'm not sure, but if not then they should.
9. An undo button. 'Nuff said.
10. Autoturrets of various types. They could range from small machine gun turrets to massive cannons and AA batteries. Maybe you could even have a MAC cannon (that might be a bit ridiculous, but whatever.) It would be nice to let the map creator choose if they are automated or player controlled.
11. A version of the visual effect objects from reach that doesn't make it impossible to see anything. That was the problem, they could have been cool, but it made it impossible to make out anything. Imagine if the pen and ink one (when combined with juicy to make it not black and white) actually gave the game a cel-shaded, comic book feel. That would be sweet.