I can't imagine Microsoft ever acquiring a publisher like EA or Sega, not only is it expensive, it'd duplicate so much administration and honestly integration would probably be hell. All the management and PR and legal departments merging with such different cultures and hierarchies, it'd be chaos that probably wouldn't be worth it.
But what I do see happening is Microsoft spending a lot of money, possibly a studios worth, to make exclusive deals with publishers like EA or Ubisoft to create either new IP or games off of dormant licenses. Imagine Ubisoft being to sole maker of Blinx the Time Sweeper games. Think about it.
In all seriousness, I see some partnerships that go beyond what we're used to with exclusive content or betas or whatever. Maybe tapping publishers to create full on new flagship IP series for platforms.
I also definitely can see Microsoft partnering with Chinese companies to potentially distribute Chinese games outside of China as Xbox exclusives (or timed exclusives), like how Netflix distributes BBC and TV Tokyo shows as 'Netflix Originals'