A way to think about it is that they're going to go for
a) games that are big enough to be notable, but not big enough to be massively expensive to license
b) games from companies that don't have their own subscription services to promote, though most of these games are also too big for point a)
A lot of times this leads people to Japanese games and indie games, which I think makes sense. I don't know if they'll ever get an EA, Ubi, AB, or T2 game to launch on Game Pass in the future, even if it's a smaller project (to the extent that these companies even make smaller projects)
If they wanted to do a really big deal to make a statement (like a timed exclusive deal?), they could maybe get a game from a publisher like WB Games. and of course publishers that put out smaller games like Focus and Embracer are in play.
but i think, in general, games from companies like Sega, Capcom, Bandai Namco are going to be some of the most likely candidates outside of the indie space.