]Most of the remnants of E3 2003 Mombasa are in the final game. There'd be no real point to putting it back in, it was never meant as a playable level for the final game. Just an ideas/tech demo.
The easiest level to add as a new one is the Forerunner Ship level, since it still makes sense canonically. The rest of the cut third act is pretty much Halo 3, so the only other one left is Forerunner Tank and Covenant Ship.
Either way if they remake Halo 2, they can't do an Anniversary style one again. Even though Halo 1 was made in 5 months, it still works as a complete game. Halo 2 today is a very obviously shortened game cut up by a painful development process.