Honestly I think Nintendo could win next gen easily. If Nintendo decided to release a console that could please the older core crowd like myself with a lineup like the 360 or PS3 has, but of course still include the Marios and Donkey Kongs that we love to play Nintendo would dominate. The hardware would need to be up to par with the other next gen consoles though. That is one of the main reasons I would never buy a Wii is because the hardware is obselete.
If Nintendo decides to play it cheap again and keep the hardware less powerful and more like this gen's HD consoles then I believe they will lose. The casual market will not move from the Wii to a new console as easily as a core market would. Therefore the gen would be up for grabs between Sony and MS.
