SURPRISE! Okay, not really.
Official Xbox Magazine UK hinted in their most recent issue that the developers of Halo 4 (343 Industries) are already working on Halo 2 Anniversary. Halo 4 is still very much in the development phase, but that does not rule out the possibility that Halo 2 Anniversary on its way.
343 Industries developed Halo: Combat Evolved Anniversary which was published by Microsoft and released last year.