UPDATE: Newegg has removed the reference to customizing bikes and designing your own tracks from its listing. It looks like the dream of creating your own Mario Kart tracks is dead.
ORIGINAL STORY: If a product description on Newegg is to be believed, then Mario Kart 8 will be the first in the series to feature a track editor. The product description states that the game "features new elements such as customizing bikes and designing your own tracks."
This statement seems to imply that Mario Kart 8 will offer the deepest level of customization in the series yet. We have been able to customize our vehicles before, but a full-blown track editor would be a huge advancement for the series.