Are they? Their last 2 games were on PSVR (Bravo Team, The Inpatient), and I don't think either of those did very well, commercially or critically. I'm speculating, but maybe that soured them on working with Sony, and they'd be open to MS.
Another reason to hope is that, if you listen to their plans, they are extremely ambitious. They have plans for 30-something different Dark Anthology games. Of course, most of those won't come to fruition, but it shows how creative and ambitious they are. They might be open to MS making them financially secure, so they could pursue their ambitions, without worrying about whether these types of games -- brief, semi-linear SP, which are kind of a gamble -- will sell enough.
Maybe. I'll keep my fingers crossed. Of all the studios mentioned so far, this would be the one I'd prefer.
The thing that makes me doubt it's Supermassive is the fact that they worked more than "a bit" with Sony, and two of their games have come out fairly recently, so it doesn't seem to square well with Klobrille's language.
I also had the thought that ThatGameCompany, the guys who created Journey, would also be a nice "get." They're multiplat now, having finished their 3 game contract with Sony many years ago. They are a small and very creative studio, similar to Ninja Theory in some ways. Their next game is headed in a direction I'm not all that excited about, but if MS is looking to build up their stable of small, creative, and talented studios, it would be a nice choice.