If they're sticking to just developers, Arc System Works or Vanillaware would be great picks, if not both. But if we're talking publishers, well, that potential opens up a lot more.
I get the feeling Sony are going for a balanced approach, so I wouldn't be surprised if more known devs and a big publisher or two are in the pipeline.
Honestly, if I were in charge of their M&A strategy, I'd do the following:
[M&A - Developers]
-Arc System Works (in-house fighting game talent)
-Vanillaware (high-quality 2D action-adventure RPG content, great fit for certain legacy PS IP)
-Deviation Games
-Ember Lab
-Shift Up (Stellar Blade dev)
[M&A - Publishers]
-Capcom (lots of valuable IP, in-house fighting game talent, big MMO asset in Monster Hunter, etc.)
-Square Enix (Final Fantasy, lots of valuable IP that could due with new installments like Einhander, Parasite Eve, etc.)
[Investments (Financial & Share Purchases)]
-Kadokawa/From Software
-EA
-Take Two
-Ubisoft
-CDPR
-Bandai Namco
-Koei Tecmo
-Devolver Digital
-Sega Atlus
-Konami
-Epic
-Pearl Abyss
Some of these companies they would obviously want to invest in more/buy more shares in than others, if the shares are available to buy. Take Two, for example, would be a higher-priority investment target than Sega-Atlus, especially if Sony acquired Capcom and/or Square-Enix. The point of the investments and share purchases is so that Sony has key investments in key 3P partners, to have some say in case competitors try to purchase these companies, among other things.