Before I begin, I just want to say I get it. Being in the game industry can be extremely tedious. Also, people get the idea that there is a shortage of ideas and that ideas are easy to just throw in a game. Also the final product has to be marketable and turn out profit for all your hard work which a lot of people who get on here to tell you how to do your job disregard. I'm here to say an idea or two hoping you'll look at it and see if you haven't thought of it yet, find it practical or if you'd want to do something like this.
Idea 1: make a game without campaign that just has multiplayer and forge and just call the game Forge. Why? So you could devote more disk space to endless forging fun and play it with friends on multiplayer, think Minecraft. You could even work out a way to put AI into forge with all of that disk space, which is what a lot of people want. Why it's a bad idea, Halo is marketable largely because of the campaign, reason why I bought Halo 4 so people wanting Halo not Forge probably wouldn't buy it. But the reason why I bought Halo Reach was largely because of Forgeworld.
Idea 2: let us play as anyone or any creature in the Halo universe, at least for funsies. Gears of war 3 has beast mode, although making a game mode for only playing as creatures isn't as cool as it being what you'd play as when your not in map edit mode.