I want ODST 2.
But instead of making it an expansion pack, it should be its own game with new graphics, and new engine, and so on. And it should really make you feel small and vurnerable. No near single handedly taking out huge waves of Covenant, no running at Spartan speed, no meleeing uninjured Brutes to death. They should just make it a game that does more with less.
And why not make it more tatical by including things like timed explosion, plantable mines, silencers that can be placed or removed from weapons, level design that gives you a 360 degree battlefield that allows you opportunity to take your enemies on from below, above, behind or head on (the first ODST did this, but I think it could be pushed even further).
And put REAL STEALTH into this game. The silienced weapons in ODST looked cool, but as far as keeping you undetected they were a joke. I think you should be able to just sneak your way past most of the game if your careful enough. Imagine how satisfying it would be to kill an entire Covenant squad in their sleep, pick off commanders with a silenced sniper rifle and slip back into cover before you're spotted, plant explosives then get to a hiding spot as a Covenant vehicule passes on patrol. Of course, you should still be able to just go guns blazing, loud and proud if and when you want to, just not feel overpowered when you do.
"I'm just a mean green mother from outer space...and I'm bad!"