Let's go through who they may have partnerships with then:
Big 3rd party publishers - not gonna happen. PS4 has too big a lead, MS has already said they are not interested in doing this (although its more they can't really afford to buy this sort of contract more than anything), and 3rd parties are making so much of their ROI on PS4 that cutting it out would be suicide.
Independent Developers - the few indie devs that are floating around that have had a previous relationship with MS, and many of them aren't doing anything with them any longer. Obsidian, Insomniac, Platinum, and Remedy are the most immediate ones that come to mind that we know are not going to be working with MS going forward. Armature could be doing a Recore 2, but going by how well I know the Xbox team & its 3rd party managers, I doubt they'd continue to pursue a franchise that sold as poorly as Recore 1 did with the developer who produced it. We already know the other project the Forza Horizon devs are producing is being published by a 3rd party, so thats not on their slate either. That leaves a few shops to potentially do a game with, including Gearbox (who are all-in on Borderlands 3), The Workshop (who have been silent for awhile now), or a smaller UK shop.
I'm not saying a small, non-publisher studio can't be working with MS. I'm saying, of the studios that fall into this criteria, who could still be working with MS? It's not like there are a ton of options out there right now. The most I could see is MS scooping up & funding some VR projects.