This is probably the coolest idea i have seen in the comments. but the problem is that in the story you are set out to kill dragons. even though there is a dragon shout that enables you to summon a dragon (which will we explained in the game) i do not think Bethesda would make something as cool as this happen.

I just can't see Bethesda putting in two years on a creature to make it a daunting and frightening enemy only to cheapen it by letting us mount them. Todd's reaction to this question was very reassuring though. It's not going to happen officially.

Flying would probably not be possible on consoles du to streaming issues, it was hard on good machines in Oblivion. In addition I think dragons in Skyrim are majestic, noble creatures that shouldnt really be ridden like a common horse. That would be like riding a lion or tiger. Even if it sounds awesome its undignifying.