I don't know if a Halo 2 HD remake would even make any sense at all, but if it somehow happens, keeping things as they were is the best solution for a remake like this. I don't know if Microsoft will eventually go for it, but whoever makes it should aim for the original design and soundtrack appeal. CE Anniversary wasn't the same on that level, the music and some characters, weapon or vehicle designs just felt out of place. I think that a 1:1 remake (just a graphic overhaul and getting the multiplayer online again) would be enough. In fact, I liked CEA's technical approach by leaving the original engine running in the background. That was such a clever idea to help the remake feel exactly like the original.
That said, I wouldn't mind if they included some specific "easter egg". I've always dreamt about playing the original New Mombasa idea that Joe Staten played live on stage at E3 2003. That was groundbreaking and the main reason I was so hyped for Halo 2. I know it sounds like an imposible thing to do, but it would be awesome if they remade in HD for the XOne that level as well, and include it as an "unlockable" extra to play. Closing some future E3 with THAT again would be awesome.
>>> Halo 2 E3 2003 Reveal Trailer