Mario's most beneficial ally has always been his prehistoric pal, Yoshi the dinosaur. But why not kick that up a notch from simply extinct to full on mythical? Dragons have been cooler way longer than Mario has, but it's about time the two forces of awesomeness had a collision.
Besides, there hasn't really been a good dragon riding game since Panzer Dragoon Orta, unless you count Lair, and you shouldn't. We think Nintendo could do much better, as long as they don't try to force players to control Mario's dragon with the Wii Remote. Which they probably would, come to think of it.