Did you ever consider a different protagonist than Master Chief?
As a central protagonist? Sure. We'd consider it, but the next story is definitely focused on the Chief. That said, games like ODST, Halo Wars, Reach and even Halo 3, with playable Elites, actually mean that the majority of Halo games, in an odd way, have included non-Chiefy protagonists.
On Xbox 360 we have not only seen the conclusion of Master Chief's (first) trilogy, but also games that have explored new directions, such as Halo Wars and Halo 3: ODST. Is this something 343 Industries will continue along with Halo 4?
We'll certainly consider new projects, although the bulk of our focus is going to be on the core FPS - there are certainly other, different opportunities to explore the Halo universe through a gaming lens. Nothing on the horizon right now, however.
Is there a possibility that Halo 2 might get the same treatment?
There's a possibility, sure, but we haven't even figured out what that content could be and how we could package it up in a way that made sense. We'll certainly give it a lot of thought, however, but it would have to be a fairly different offering than Anniversary.