I'm going against the grain here and saying no they won't be going third party, at least any time in the near future (at least 10-years). Someone said that consoles don't make that much sense anymore, but sales of home consoles prove otherwise. Somy's gaming and home console market is one of their few markets that have kept them alive, while MS still makes a boat load off the XBONE despite it lagging behind. Nintendo released a stinker with the Wii U, and while some say, "they can't fail again or else" I think they don't realize the war chest of money Nintendo is sitting on. They've only had a handful of blunders in terms of games and consoles over their lifespan, and many seem to forget that their biggest financial success, the Wii, just happened not that long ago. Nintendo has so much money that they could literally have 3 more failed consoles in a row before there was serious consideration of needing to close, sell off IPs or go third party.
With all that said, I highly, highly doubt we'll see a third party Nintendo for quite some time if ever. I don't see them abandoning consoles or handhelds anytime in the near future, and think that as long as there isa large, profitable market for them, they will continue to develop new consoles to release their games on. I'd go as far as to say that it may be over 15 to 20-years before Nintendo ever takes a step in the third party direction.