In the interest of speculation, I think the question is more fun when stated as
"If Nintendo decided to go on a spending spree, which developers would it be most advantageous for them to purchase".
I mean, obviously due to size fan favourites like Sega* and Valve are right out due to being very expensive. Rovio would be pretty interesting, but the price tag would be pretty hefty, likely much higher than the company's current value. I'd love it if they bought that UK division from Crytek, since they have positive history with Nintendo and are
. It would be epic if Nintendo basically farmed all the big recent Kickstarter successes and nabbed them on the cheap, as they'd end up successful even if a small minority of the purchases ended up with solid sellers (also Super Mario Crossover the eshop, woohoo!)
I don't know if Sega is off the table. It's argued that Sammy bought Sega not for its IP catalog or its developers or even its publishing business but for its arcade distribution and entertainment center business, which it had a vested interest in as a pachinko company.
If Nintendo came in and basically let Sammy keep Sega's publishing arm to do as they saw fit with it and all of the arcade business, but took everything else... y'know, the part that Sammy doesn't know what the fuck to do with anyways, I think they'd agree to those terms, especially since
Sega Sammy's "home entertainment division" has been losing them the most money while all its other business branches post MUCH better numbers. Make for an easier sale than you realize.
Wait a minute. Didn't Iwata say during a past interview or shareholders conference that they weren't going to buy any more studios outright? I'm pretty sure I remember him saying that Nintendo now prefers to go into publishing deals with other developers (like how they're publishing P-100 for Platinum) rather than buy them outright in order to avoid talent leak like when two of the main Metroid Prime guys left Retro.
If that's how it is, what I'd really want to know is what other developers (i.e. Western) would it be best for Nintendo to enter publishing deals with?
I've said it before, and I think the big thing with Nintendo is that, if such a thing were to happen, they would do it if the talent they're acquiring is actually on-board with the move, like it was with Monolith Soft. From what little you hear from them, they basically retained almost the entire team in the changeover because all of the design leads WANTED to make it happen.
Nintendo is also a different animal in Japan than it is here and is greatly respected in the game industry there, which is why you see things like the Namco Bandai Smash Bros. collaboration. Nintendo is a company that new talent wants to work at, as well, which is why they have the lowest staff turn-over out of most, if not all, of the other devs and publishers in Japan and get their pick of college grads.
So yeah, I think they could and should pick up Sega. Platinum and Mistwalker aren't out of the realm of possibility, either.
The trick is, of course, Nintendo wouldn't act on them unless the talent wants it to happen, so it's a kind of wait and see approach.