This would be awesome. I'd pay more than 800ms points for that but I don't mind. :)
I would be really cool, if not only they have the in-game Forge, but a standalone map editor todownload on your PC or something that had a simplified version as to not running the game engine, for lower end PC's, but have advanced controls like the things mentoined here and more. Then, you could upload your map here to BNet, or even connect you computer and 360 via direct ethernet, or a network/wi-fi hotspot, and the game could have compatability with the editor to tet out your map and save it to your 360. As a supreme development suggestion, they could have it like Forge n XBL and you could link the editors to other people so you can share suggestions, and show them on-screen what your talking about, then undo it on your side to the point before you showed them so they can think about it. Maybe even have your 360 on at the same time and see the live updates of you moving and changing settings in the PC map editor.
Of course, all of this awesomeness wouldn't be Forge, as we are talking about Halo 3's in-game Forge.
Sorry, to drag that out but, yes this is definetly a great idea. Also have limits to the amounts of NPC's like objects, and have a host to the game to set how much budjet each player may use, and what type of items they can spawn and where they can spawn them.
I've got a lot of other ideas but maybe I'll save those for my own topic, to save everyone here th trouble of drying out there eyes on this single post.