Haven't really been following the news surrounding the next-gen hardware, but this nVidia Shield seems more akin to the PS Vita than to traditional consoles. Maybe the handheld market is where the Big Three's position is weaker than it's ever been and a chance to have success is greater in that area. It's an interesting move by nVidia, but, I dunno, a bit too much like this
Anyway, I'm not sure the 'Big Three' are being challenged with these upcoming projects, I think the gap is too big at the moment, but I do feel there's room for more than those three in the console gaming market and the Steambox, especially, seems to make a move to get a place between those consoles and has a chance for success (though I'm not sure what to expect from it). Then there's also Game Stick...
I guess the more the merrier, hopefully some of these actually turn out decent.