I don't really think the answer is to pull a 'Disney' and go nuts & purchase all these studios, rather than to expand upon the ones they already have.
Particularly EPD, Monolith Soft., and Retro.
Compared to.... say Naughty Dog, who currently has 300 employees and they put games out at a relatively good pace. Last of Us was 2013, and Uncharted 4 is next year. That's a 3 year gap between games, pretty reasonable.
Now look at Retro, which is around 100 employees, DKC Returns and Tropical Freeze had around a 3 year gap as well. Pretty reasonable, however there have been demands for years about them doing Metroid, or a new IP entirely. But if they were to grow around the same size as Naughty Dog, they'd be in a better position to put out 2, maybe 3 projects that can be DK, Metroid, and/or a new IP.
As for Monolith, seeing as how long it took them to make X after they finished development for the original Xenoblade Chronicles, it could be another 5 years before we get Xenoblade 3, or whatever new game they come out with on NX, which would be well into its life cycle at that point, and right now Monolith sits at 123 employees (as of last year, I'm going off Wikipedia). Having double the staff could lead to them putting out these giagantic, groundbreaking RPGs a lot faster, and could enable them to grow into Nintendo's BIGGEST asset outside of the Pokemon Company.
And EPD, which has merged EAD and SPD together, means that they've got far more developers working together one multiple projects, which should lead to faster development. A good first step, but I think that adding an "EPD Group 2" to go along with Group 1 could go a very long way into shortening development times and increasing effectiveness when it comes to releasing and making HD games.

That's really what I think they should do rather than try to buy out any 3rd party companies... Although I wouldn't be against them buying Intellectual Properties, like say... cough*Sonic*cough.

And to think:
If they were to expand many of their development studios to both decrease development time and increase software output, ALONG with the potential, most likely idea of NX being one hybrid console-handheld platform.
All of those expanded, bigger studios would only have to concentrate their all their efforts and resources on one platform, rather than divide and seperate them into 2 different ones, which had led to all the software droughts we've seen from the Big N this generation, ESPECIALLY on the console side.
Doing all that could wipe out the software drought problem almost entirely.

Capcom, Sega, Platinum and Level-5. Especially now seeing as the former two are falling down anyways. Level-5 is a great developer and Nintendo needs to keep them exclusive. Next Level, Playtonic and some Western studios would be a wise investment too. Nintendo still has some of that Wii/DS cash, they should be bloody well using it.