Well I've answered a topic like this before, and here's what I had.
Make a strategized third person shooter game, thats both gritty, and challenging. Basically combine Tom Clancy's Ghost Recon Future soldier gameplay mechanics with the Halo universe. Hopefully without all of the glitching and a bit more strategizing, and A LOT more use of equipment and choice of how to take on certain situations. That was my dream when I first heard about Halo 3 ODST when it first was announced.
I wanted to see a halo game that took more thinking, with the strategy of a team of regular soldiers, or early stage spartans, going through a technically fragile situation that reqiures the player to both plan ahead, utilize the tools and environment available, to enter a state of play that is variable upon your choice to handle the encounter and accomplish the missions.
Teams of ODSTs or Marine Veterans would be very interesting, perhaps they could be doing The ONI spook work like they were in Glasslands. I could definitely expand on this further, and honestly, coming up with some very good Halo characters and a decent storyline wouldn't be too hard, it would just take a bit of thinking and some collaboration. Then get it into the gamestage, always make the player feel like part of the team, and make them feel like they know who they're working with.
ODST was a good idea, it just needed to be expanded and worked on a little longer, this would make ODSTs point not to have been lost in vain, because I loved ODST for what it did for the characters, it just needed more.