Seriously though, Microsoft buying SEGA would be a massive boost to their portfolio, especially in the Japanese market that they've traditionally struggled in. Most of SEGA's output for the last 10 years has almost all been day-and-date with PC anyway, so even their goals are already mostly aligned anyway. Not only SEGA proper with it's Sonic the Hedgehog games, but having Creative Assembly games on Game Pass day one would be a massive disruption in the PC space. Microsoft already worked with them to do Halo Wars 2 a few years back anyway... Along those same lines, Relic Entertainment (also owned by SEGA) just made Age of Empires IV for Microsoft. Amplitude also mostly is doing PC games (Endless Space, Humankind) and would be a good fit as well.
Atlus would be a megaton if Microsoft acquired their parent company. Obviously Persona 5 needs a PC port, badly. But buying that kind of console exclusivity for games that have traditionally not gotten either PC releases or Xbox releases, that'd be a big win for them. Xbox still has the stigma of "if you like JRPGs, don't buy an Xbox" and the acquisition of Sega and Atlus (along with already acquired Bethesda) could really change that perception.
Let's see, what else do they got? Sports Interactive: again Football Manager being part of Game Pass would be huge for it's rabid fanbase. Yakuza, Valkyria Chronicles, Phantasy Star, etc.
I honestly wonder why Microsoft hasn't plunked down the cash yet.