Timing and doing 'the right thing for the franchise' are key
Microsoft Game Studios VP Phil Spencer tells IGN that the rumored Halo 2 Anniversary is still being talked about internally. While it hasn't been officially announced yet, such a re-release seems inevitable. "I'll be open," he said. "We have discussions about it. It's more about the right time and making sure that we really would do the right thing for the franchise."
Elaborating, Spencer went on to say "The Halo Anniversary that we did was as much about the launch of the first Xbox as well. It was a culmination of many things coming together. It would probably run out of steam if we were trying to think about, 'Okay, what's 10 years old now?' It's definitely more from the game itself, the experience that we’re able to bring to market. Less about 'Is the last digit a zero?' in terms of its anniversary."
Long-time fans would probably be down for another Anniversary release assuming we don't get overloaded by new Halo titles in the meantime.