... most of the Ensemble folks I'd have headhunted for the job would be the environment and concept artists. The work those guys did was golden.
*Digs around in his hard drive*

Won Choi designed the Forerunner ruins on Arcadia and the Shield World, as well as some of the UNSC buildings.

Phil Wohr designed some of the Covenant structures. In practice, most of his designs were pretty fugly, but with current-gen graphics... watch out!

Matthew Burke was the greatest concept artist Halo ever had, and it's a shame that 343i hasn't hunted him down yet.
Perhaps you're familiar with this Sparrowhawk concept art?

Or this?

Or perhaps the data cards of the UNSC vehicles? He did most of them.

Pair some of these guys up with a development team that's working with the Company of Heroes engine, a writing team that includes Eric Nylund, and ship the game PC-only with mod tools.
Game of the decade, right there.