Studios that almost or entirely exclusively work with Nintendo like Next Level Games, Shin'en, Grezzo, and AlphaDream are no brainers.
I also think Level 5 and Platinum Games would be great acquisitions. With Level 5, Yokai Watch and Professor Layton might as well be 1st party Nintendo franchises given how those franchises have appeared exclusively on Nintendo systems to the best of my knowledge. Inazuma Eleven is also very Nintendo heavy. The big one hear that would be a shocker would be Ni no Kuni. That'd be a tough pill for PlayStation and PC fans to swallow, but it'd be a good gain for Nintendo and that's the point. As for Platinum Games, if there was one company that would make the most sense for Platinum to be bought by, it's Nintendo. They have a very strong and healthy working relationship already, the strongest one Platinum has at the moment, Square Enix being not too far behind. Bayonetta is essentially a non-official Nintendo franchise at this point. She's almost on the same level as Samus in terms of strong female characters on Nintendo. Plus, if it means more off the wall, compelling stuff like Wonderful 101 and Astral Chain, I'm all for it.
And that's it in terms of acquisitions that I can think of. Outside of that, I think the best approach would be to grow and expand upon the studios they already have.