Personally I prefer halo 2's PC forge mechanics beyond anything else, but love the selection of props they gave us in halo 3. If there was a halo game that was just dedicated to forge, sort of like an off branch title, it'd be my type of game.
No forge budget limit, thousands of set pieces, terrain, materials, buildings, scripted objects for you to chose from
Imagine distinct maps that are 10km x 10km, like a desert, ocean/island, forest, mountains, and space.
Imagine loads of various vehicles and weapons from the UNSC and Covenant, from the game AND the novels. A multitude of vehicles to chose from.
32x32 or 64x64 vs games involving ALL of the UNSC forces against the Covenant.
Script and edit missions, make ai battles, change the lighting and scenery with tweaked forge mechanics.
Create a custom campaign or make the best damn map of Halo's history.
Being able to play as a Jackal sniper in the cliffs or dropping down out of orbit as an ODST.
I've been dreaming about a game like this for so long. The only logical thing to do would be to recommend this to someone higher up @343 and hope for a response, chances are either way they'd either come to a decision that "that's a bad idea" or "we don't have time to copy assets from our game and paste it onto a previous engine and add multiplayer support because we're too busy killing the halo franchise".
;_;