Age of Empires?
Microsoft plans to begin working on a new strategy title. Not that it made a public announcement or anything, but a job posting on LinkedIn outs its intentions. The position is for a team lead on a "new AAA title within an established and well-loved strategy game franchise."
Right off the bat, this screams Age of Empires as it's been almost three years since Age of Empires Online launched. That's the safe money, but don't discount the idea of another Halo Wars. It may not be "established", a "franchise", or even necessarily "well-loved", but Microsoft's shown a willingness to do almost anything with the Halo name lately.
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