Looks like 343 Industries may have more up their sleeve than just Halo 4. According to OXM UK, they're following up the well received Halo: Combat Edition Anniversary with something similar for Halo 2.
Released last fall, the enhanced version of Halo: CE added upgraded visuals and new industry standards like achievements, and featured a multiplayer built on the Halo: Reach engine with remade maps spanning Halo: CE, Halo: CE PC, and Halo 2. Likely, Halo 2 Anniversary will do something similar, though who knows how its release might coincide with Halo 4 coming up on November 6.
Do you have any interest in revisiting Halo 2?