We NEED a remake of Halo 2. It's the only game besides CEA that needs a remake. Halo 2 started first-person shooters! It totally needs it. The graphics are nothing compared to Halo 3, Halo Reach, and Halo CEA.
As for Halo 3 now, i think we just need an update. Giving better graphics. A remake would be totally un-necessary. An entirely new game? Just no. An update would bring back players to the Halo franchise. Better graphics, new remastered maps, it's all we need. But now that you think about it, why dont you just put in that feature in Halo CEA. How you can switch back to old graphics, and back to the new. It'd be sick and I, personally, would go back to H3. It'd be totally awesome and epic.