Which the launch of Halo on PC, I noticed that you don't have to buy every Halo title. I thought that's great so we don't have to pay for a game we don't even want. I think this also gives us the possibility of adding Halo 3 Anniversary along the way but as a DLC, much like how each game has been sold separately. I'm not sure how they could do it for people who already own Halo 3 on MCC besides making Halo 3 Anniversary a completely separate title. But all 343 needs is a high enough demand for it and make the DLC a fair price and it will make profit for them. This is coming from someone that thinks Halo 3 classic is masterful, but deserves the attention of an anniversary edition. Definitely bring back Sabre and Blur to work on the campaign!!
Halo 3 Anniversary could still happen