Personally, I would be happy to see either a small-scale ground game, a 28mm skirmish, or a fleet game. However, they'll have to stretch the Halo universe regardless of which direction they take it--if, indeed, that's what this actually is. The UNSC forces were fairly limited in different types of personnel and ground vehicles, and we didn't see a whole lot of the space fleets on either side. Now, what we DID see was pretty cool, and it wouldn't take much to expand on it, I don't think.
Also, there would be an issue of timing, especially if they go with a 28mm-ish skirmish game. Depending on when the game is set, there would not be very many SPARTAN soldiers--the UNSC player would be limited to marines and ODSTs (and hopefully the marines would be more effective in the miniatures game than they were in the video game!)
Eh, I sound like a Halo fanboy. I guess I am. I mean, geez, I'd buy the models just to have them!