As to what unannounced strategy game it could be, it would seem that it clearly is either Age of Empires or Halo Wars. The job postings don’t seem to provide any real clue as to which franchise it might be, but it does appear that it will be coming for both Xbox One and PC, when the title is finished.
Microsoft’s former strategy game studio was Ensemble Studio who created both Age of Empires and Halo Wars. Microsoft closed Ensemble back in 2008 when Halo Wars was completed. Since then, we have seen a re-release of Age of Empires 2 for the PC back in 2013, but beyond that we have heard nothing more. We suspect however that we should hear something more specific about Decisive is working on. Perhaps an announcement at E3 later this year?