Theres some thinking that Nikon might produce another top of the range with a crop sensor as the current 2 top models are so close to each other, they both use the same sensor so it might make sense to bring a crop back into the top line.
Having said that, Nikon are streets ahead of Canon in the high ISO business and have only got so far in front because their best sensor is full frame, im pretty sure a crop sensor would only make the iso sensitivity worse so who knows.