If Halo 2 were to be remade it should be like this:
1: Disc 1 was campaign and Disc 2 would be Halo 2 Multiplayer HD.
2: Campaign would be sold separate from the multiplayer.
From what I remember Halo 2 was the most played game on xbox live until Microsoft pulled the plug on all original xbox games supporting xbox live so there should be an actual remake of Halo 2 multiplayer