Mistwalker would not happen. MS tried to break into Japan with third party moneyhatting and it didn't work. Buying a studio is a much more long term thing than moneyhatting a few games, and thus carries more risk if it fails.
Infinity ward would be a terrible decision. They have never made a game besides Call of Duty. They would have to be be bought for a lot of money, bring the rights to 0 IPs with them, and they are far from the best developers that MS could buy.