It might push a million Zunes or so, but I would be worried about what it might do to the perceived value of the Halo brand.
In my opinion it would be seen as somewhat "jumping the shark," indicating that they don't really know what to do with Halo now. It could even end up being a fantastic game, but it wouldn't sell nearly as well as a real Halo game, and would just end up being used as flamebait by people who had never played it.








