I would go for catalog completeness. My goal is that Ms would have top tier talent with the industry best on every single region so they could produce games among the best of the industry in every single genre.
So:
Japan Games:
Mistwalker
Grounding Inc (Futatsugi, Panzer Dragon Creator)
Itagaki
White Owls
Yuji Naka
Perhaps one bigger studio in Capcom/From/Level 5/Platinum.
That team would give them the ability to create JRPGs, fighting games, action adventure games, character adventure games, and they have produced some of the best games in those genres.
And for Yuji Naka: BALLAND Wonderworld! I mean who wouldn’t want to have this under their umbrella?
Narrative Driven Games:
Dontnod
Asobo
Bloober Team
Dontnod and Bloober team are my favorite developers for history games this generation in their respective takes (bloober being obviously more terror focused). Plague Tale while the least favorite of the games I’ve played from this trio already oozes potential for a first attempt as well, not to mention how incredibly talented and diversified the team proved to be in all the genres they tackled.
Isolated mentions with commentary:
Techland. They are able to make dense open world games with a nice gameplay and with up. To 4 people coop. Imo that suits really well into Ms offerings and they could be a great asset to help more games achieve those goals. Imagine for example they mixing with platinum to make the 4 player coop large open world with flying islands dream a reality?
Not to mention that they make good games too. Dying Light was very good and kept me and my brother playing together until we finished it completely.
IO interactive:
Imo,right now these guys own the stealth genre. And they are very efficient too. Even if Ms doesn’t get Hitman when purchasing them Ms has Perfect Dark that I feel would be perfectly suited for them, not to mention it would be nice to see what they can come up with new.
Bungie:
Well, can’t leave that one out, can we? Bungie craft for shooters is almost unmatched. The one thing that you can absolutely be sure for any game they make is that you know it’s going to play like a dream.
Having them back creating new stuff like Destiny was, as well fully supported so we can keep getting all destiny content on gamepass would already be worth it. There’s also the argument that once XGS reaches a certain size we’ll definitely see multiple teams tackling a single game. And who wouldn’t want to have Bungie in that pool?
Sega:
I’ll never stop rooting for it. Sega just makes too much sense for Ms.
Sega of Europe has some of the most famous pc only games at the moment, with Total War, Company of Heroes, Football Manager… And there’s a possibility to bring those to console as well having them supported and with Gamepass to find that audience. With Age and those developers, Ms can have the rts fans locked into gamepass, and build an audience on console to increase that even.
Then there’s their lineage in Japan. A way for retail to accept Xbox consoles, long term relationships with hundreds of legendary developers and those relationships could be taken advantage into growing Ms efforts in the region. (notice how many of the Japanese developers that worked with Ms already came from Sega)
There’s also Atlus that it’s one of the best jrpg developers at the moment that could at least start developing for Xbox and releasing them on gamepass.
Edit: THQ Nordic would make a damn nice purchase too. They are swimming in IPs and nice to have all the talent secured, but their approach of only making smaller games is hurting their potential.
With the increased investments enabled by gamepass that could become huge. And adds even more unique portfolio. Darksiders (which has been branched into DMC, Zelda and Dark Souls like) euro Rpgs would also would drive gamepass and Xbox adoption through Europe. And many more.