I know there's a perpetual 'MS is buying Sega' rumor, but I think with recent financial reports taken into consideration, my money is on SquareEnix. They have a great portfolio of games and franchises, including all the old Eidos stuff. Tomb Raider, Deus Ex, and most importantly, Legacy of Kain. Oh yeah, and Final Fantasy and Dragon Quest.
If they do acquire SquareEnix then Microsoft would be producing more PS5 exclusives than Sony, when combined with the Zenimax Studios exclusives.