I think the most important thing Nintendo could do is offer the license to a physics engine for developers from day one, one which would allow easy interaction to a 1:1 controller. I still don't think that "better graphics" are the way of the future very far beyond the Wii. Truthfully, I feel much of what's in high-end titles is excessive.
So yeah, instead of focusing on graphics, polygons, textures, and stuff like that, I'd hope Nintendo focuses on higher computing power based on phsyics. A Golf game that literally recreates the true physics of golf can be next. The same with a tennis game. Sure, we get a lot of that now with the motion plus, but imagine things growing "more real" in that sense. The PS3's isn't any better than the Wii with the motion plus, from what I've seen, so I think this would be the direction Nintendo might move in.