Hmm, I don't believe that Certain Affinity is making Halo 2 Anniversary alone. If they are making it, they will probably make it in conjunction with Saber Interactive and 343 (design only), just like Halo Anniversary.
Campaign
Redesign some of the levels to be more open and have a wider scope
Use the Elite and Brute AI found in Halo 3 & Halo Reach
Get rid of 1-shot Jackal Snipers
Increase the background warfare (buildings collapse into the distance and dogfights in the air)
Include another Earth Level, another Halo level, and another ending level that bridges the gap between the orginal Halo 2's end and the start of Halo 3.
Multiplyer
Get rid of ALL the glitches.
Get rid of BXR.
Eliminate noob combo by depleting Plasma Pistol Charge
Reduce Sword Lunge
And Halo 2 becomes pretty much perfect. There is no need to be 100% faithful to the original Halo 2, it's campaign isn't as revered as the original Halo's was, and improving multiplayer design flaws is just accepted practice.