How Halo 4 Can Revitalize The Franchise

Officially, Halo has been a trilogy. There have only been three numbered games in the series, despite an RTS spin-off, a side-story that followed an Orbital Drop Shock Troop squad through a Covenant-occupied New Mombasa, and the prequel (yet, ironically, last of the series to be developed by franchise creator Bungie), Halo Reach. Largely first-person shooters, sharing the same core mechanics, what distinguishes a "secondary" title like Halo Reach from its numbered brethren?

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