Could it be that Microsoft has finally realized that three years between Halo releases is too long? It looks like more frequent releases in the Halo franchise are going to be in store for the post-Halo franchise after Bungie.
The company does not apparently have a strategy yet on how often we will see new offerings in the Halo franchise, but they are looking at it. Although Microsoft released Halo Wars and Halo 3: ODST, the last previous Halo release was Halo 3 way back in 2007. It does seem that players want more Halo titles on a more regular basis.
Sources says that while Microsoft has not ruled it out, it is unlikely that we will see yearly Halo releases; but a move to every other year might be in the cards, as there is a lot of ground still to be covered in the Halo universe.