I would like to have Halo Reach: ODST. I want to explore Reach with Buck and Dutch. You could even play at the part in which Noble Six leads Buck to safety.
For Armor Abilities:
Sprint
Jetpack (like the Bullfrogs)
VISR Mode (It would be better as an Armor Ability; It would enhance your vision, give you a map, and have a feature in which you highlight an object and it gives you intel on said object.)
Night-vision would act as the brightening feature, being available at all times.
I'd like ODST customization; Dare's helmet, Buck's chest, Mickey's backpack, etc.
Also, I think It'd be cool if we could see Rookie get hired into the squad. Give him a little backstory, ya know?
I like the tactical combat thing, but for multiplayer? I'm thinking that'd be better for firefight, like completing objectives. (Example: hold a certain area, assassinate an Elite General) It'd be compatible for "Battle Bro" as well. And, I'm as much of a Bulletstorm fan as I am Halo fan, and I LOVE Bulletstorm's Echo mode. If any Halo game would work for an echo mode, It'd be ODST.
And as for Harvest....I think the best way to experience that would be to play as a Marine. Yes, I just said that. Marines were the first to encounter The Covenant, so it's only fitting. And don't forget...Sgt. Johnson was the first to kill a Covenant soldier. If I could play as him, and get that first kill, I'd be the happiest camper in the galaxy. :)
So those are my thoughts!