Who are we considering as Nintendo's former Western development stable? Left Field Productions made a few decent NBA games as well as Excitebike 64. Does Rare count? I always consider "West" as referring to North America. Even if we consider Rare, we shouldn't ignore how much hand-holding they required to produce results. Who knows what the status of Silicon Knights is? They made one really good original game (with heavy involvement from Nintendo) and rode that horse for the next decade before falling so tremendously. Are we counting Factor 5 and Angel Studios (now Rockstar San Diego)? Were they ever officially part of Nintendo's stable? I'm sure TJ or Luigi Dude will do the research since I'm too lazy. Nintendo Software Technologies should just be renamed Mario vs Donkey Kong eShop Studio.
Retro Studios is the best Western team ever associated with Nintendo. Unless you count Factor 5, they're essentially the only Western team to produce results on major projects with any kind of autonomy and even then, they've lost a lot of good people. Sure, they replaced those departures by hiring talent from Naughty Dog, id Software, and Vigil Games, but I have to wonder if the new people will mesh as well as the old or if we'll end up with the 2012-2013 Los Angeles Lakers... This season was brutal.
As far as I know, Platinum Games is technically available in that they're an independent company that
could join Nintendo. Their only announced projects are for Wii U. Company president, Tatsuya Minami, is already on record stating that they will be making PS4 games. Acquiring the company would only be worth it if they wanted to become a Nintendo first party (no point if, say, Atsushi Inaba and Hideki Kamiya up and left) and Nintendo let them just make whatever they wanted. If they were just handed a bunch of Nintendo IPs to handle, that would be a colossal waste of their talent. Not saying they couldn't make a good Star Fox, F-Zero, or Metroid, just that forcing them to work within the confines of existing franchises they didn't create themselves is limiting for such talented game designers.
I'd hate to have this hype for this unknown thing that could possibly be a predictable given.
Manage your expectations. Neither Nintendo nor Retro Studios are responsible for the hype the media and gamers are creating for this mystery game, unless you count silence as hype. They've said very little other than they've had Wii U development kits and are working on a Wii U game in addition to a few off-hand comments that people blew up.