If they want to have any sway at all in Japan it would have to be a major developer. I think Capcom would be the most impactful and they are currently sitting on some hot properties.

Generally speaking, companies don't buy other companies that are poorly managed.  They might buy some IP from Sega though.  

I think Nintendo would make the most sense, except that I doubt their corporate cultures are compatible.

The Pokemon Company

The solution to Microsoft's Japan problem is the same to Nintendo's Europe problem, a unified software support plan. No, I'm not talking about one buying or merging with the other (that's unrealistic), but both of them releasing software (1st party) on the other's devices, even if it takes some reworking. I garuntee Microsoft would spend a butt load of money to optimize the tar out of Halo Infinity to Port it on the Switch if it also meant Super Mario Odyssey came to Xbox. Both companies software sales would explode, particularly in markets where their hardware struggles. The profits would soar and greatly offset whatever magic they'd have to use to Port ALL their 1st party games to the other's device. This is the future, and I feel like both companies have been moving this direction for quite some time with Minecraft, Super Lucky's Tale, and Banjo being in Smash. Now I'm just waiting for Nintendo to put something on Xbox/PC. Seriously, whether in my scenario or one of your own, just imagine if you could buy all Microsoft and Nintendo games on one device. That would be insane. I'd include Sony in this, but I think they'd sooner die as a company than do this.

Honestly, as much as I don't want Microsoft going on a studio buying spree instead of creating their own talent, Konami would be a decent purchase. Microsoft is going for a more multiplat-friendly attitude, and maybe they could do something about all of the IPs Konami is holding hostage. I don't see it happening, though, Microsoft might not want a company deep in the gambling scene (and all of the criminal ties that brings up).

None would be the best scenario.

But aren't their laws in place where Japanese companies can only be bought by other Japanese companies over there?

Tango Gameworks is owned by ZeniMax an American company.

Sakaguchi team ?

In any case, this will be sad news for particular people.

Konami. They might cancel each other out.