An alliance with Valve would be my choice.

Platinum Games,and Grasshopper

Hack N Slash titles should not be exclusive to Nintendo. Thats the smallest market for it to sell. You will see with Bayonetta 2.

I doubt Grasshopper or Team Ninja would be happy working exclusively for Nintendo as many of their more sexualized / hyper brutal games wouldn't resonate well with the image Nintendo wants to convey.

Yes, Nintendo did pick up Bayonetta 2, which has both of those elements, but I can't see them supporting torture,brutality, wobbling breasts and obscenities in the long run.

Nintendo buying companies to make their brand seem mature, is like Disney buying Lucas Arts and Marvel to change their public image.

Killer Instinct, Conker's Bad Fur Day, Perfect Dark, Eternal Darkness, Geist, Zangeki no Reginleiv, Fatal Frame, Ninja Gaiden 3, Bayonetta 2, Pandora's Tower... Nintendo has no problem with publishing these kinds of games nowadays. They just don't dedicate themselves to them, for a variety of reasons. If they had one internal developer dedicated to that, their overall output wouldn't change much. And I don't see them buying a developer of this kind just to censor it... They certainly seem to give Monolith Soft a good amount of freedom to do what they want.

Regardless, I don't see Tecmo-Koei letting Team Ninja go anyway... They're too important to them. But I don't think it would be hard to get the Ninja Gaiden or Dead or Alive franchises to be WiiU exclusive, if Nintendo cared to do so. Remember Resident Evil ? Why would these be more difficult than that ? The thing is, it's probably not worth the hassle, giving their current sales.

Anyway... Platinum Games, Mistwalker and Treasure would be my choices, but I don't see any of them accepting the offer as they seem to be quite happy with their independence... And maybe that's for the best. As others already mentioned, creating partnerships with them could be good enough for both sides.

On a related note, I can see Good-Feel becoming a 1st party studio in the not so distant future... They seem to fit right in with Nintendo's internal developers.

I can't see Nintendo buying Team Ninja, without buying the Tecmo-Koei whole, which actually would be a solid investment if only for their IPs if the company ever was near going under.

If Nintendo were going to buy a whole publisher, I don't think Tecmo-Koei would be the first choice.

I actually think Sega would be the obvious choice. They have some IPs that have good alignment with classic Nintendo franchises, and then they have franchises that fill gaps in Nintendo's lineup. They have similar basic ideals, but Sega doesn't have an equivalent of Miyamoto to enforce high quality.

And Nintendo would easily be able to convert Sega into a special brand for games that aren't typical Nintendo fare.

Who: Next Level Games
Why: Developers of Luigi's Mansion: Dark Moon, Super Mario Strikers (& Charged), Punch-Out!! Wii; more Western devs would help
How: Eh, with money?

==Alternatively==

Who: Wayforward Technologies
Why: Western dev with experience making Metroid-esque games and developing for 3DS; also secures IP like Shantae and Mighty for Nintendo; give them Metroid and tell them to go nuts
How: Again...with money? Not like they're buying Disney here.

The "how" doesn't so much refer to the question of money, but how they'd go about getting into the position of buying them.

After all, it's not just a matter of walking up, handing over money, and adding the studio to the books. Why would Next Level Games sell to Nintendo? How would Nintendo encourage NLG to sell the studio to them?