I can't see them buying anything too big, so the people suggesting they buy whole publishers above, that seems highly unlikely.
Possibilities as I see it:
-Platinum (MS could revive Scalebound and earn alot of goodwill for doing so. They could allow them to keep releasing Bayo games on Switch as well as Xbox to earn some good will with Switch fans)
-Mistwalker (MS literally founded this studio. Lost Odyssey 2 would be huge. Only problem is that Mistwalker is quite small, they are a concept studio, MS would need to find a 2nd studio to do the development legwork)
-Grounding Inc. (MS could revive the Phantom Dust reboot. They could make the Crimson Dragon JRPG that Yukio Futatsugi wanted to make. They could make Crimson Dragon 2)
-White Owls (Swery and MS have had a pretty good relationship in the past. Would be truly epic if MS bought the D4 IP from Access games so that Swery could finish D4)
-CyberConnect2 (they are quite Xbox friendly compared to most JP devs, with all 3 of their upcoming new IP's releasing on Xbox One)