I can't see Nintendo going third-party anytime soon, even with the Wii U struggling the way it is. Despite their difficulties their still producing their own games for their own systems and still making money.
They'd be amazing at being third-party, though. Their ideas for hardware are often kind of out there, but games? They've IPs and plenty of creative talent, and recognise a good idea for a game when they see one. Splatoon, anyone?
But, I have to admit I'd rather see them do their own thing on their own systems. Nintendo may be weird and out of touch with several aspects of reality, they do bring new ideas to the table. They introduced touchscreens to the gaming masses, figured out affordable 3D on a handheld, introduced many people to gaming with the Wii's motion controls and even the Wii U's GamePad is kinda neat (and convenient if you live in a house with multiple people and just one decent tv). The industry wouldn't be the same without their brilliant weirdness.