At this point, if they are going to snatch up more studios, I would hope they would be small teams that could greatly benefit from the increased budget.
As mentioned already in this thread, INTERIOR/NIGHT would be a good get, they just partnered with Xbox to release As Dusk Falls which is a great game for a subscription style service. If they could release one of those on a more consistent cadence with a little more budget it could help fill the gaps of 1st party while the bigger players work on bigger projects.
I also think White Owls would be a great choice. MS is making a push for more Japanese content and Swery had always seemed to have a good relationship with Xbox (Deadly Premonition launched as Xbox exclusive in the west, so did D4, The Good Life launched day 1 on Gamepass). The main problem with these games is clear lack of budget and polish but they're some of the most unique games in the industry, being supported by a subscription model could really help them while giving Xbox some exclusive Japanese content to compliment Tango Game works.
Certain Affinity also seems like a slam dunk, they have already been tapped to help with Infinite and have a long history with Microsoft. Sounds like they want to make their own game too, smaller scale projects that can release more frequently is exactly what Game Pass needs and all the better if it's from studios they already have a great relationship with.