The 100% Completion achievement description in Halo Wars reads "Ready for the Sequel", hinting that developers from the now defunct Ensemble Studios might have known they'll work with Microsoft again in the future.
The VGB writeup indicates that Robot Entertainment can team up with the internal MGS team, both made up of former Ensemble employees, since they are already working together on Halo Wars support and DLC; to make Halo Wars 2 at some point in the future.