I'm not sure about a whole world, but what about a small zone, or maybe a building you go in and everything is old-graphics? Like in Paper Mario, there's a pot in Boo's Mansion you can jump in that will turn you 8-bit and play the Star Theme while you're transformed.
I'm not saying a throwback world wouldn't be cool, I just really love transformation powerup gameplay mechanics and I think transforming into an 64-bit model (much like how Paper Mario had Mario transform into an 8-bit sprite) would be a really funny and really fun powerup.
I find the nostalgia thing odd but I'm definitely feeling it. Why do low polygon models and very digital songs please me so much while I certainly wouldn't want to play anything with low FPS. I don't necessarily think it has to be low polygons just reminiscent of it. Think about the Xbox live versions those are high definition but still feel extremely classic.