So people that like fast-travel can still basically do it, maybe a bit slower, but also get to ride a DRAGON!
And those people that don't like fast-travel because you miss out on the scenery can also get around on foot, but at times, when they need to move across the whole of Skyrim, they can enjoy the view, instead of watching a loading screen.
Of course, it must be balanced, so the dragon won't be a very effective attacker, maybe a weak fire-breath at most.
And they should be summon-able spells, of which you can buy scrolls from, and you can also use Conjuration to summon one.
But you can also have permanent ones you buy from a stable, and while not as fast as the conjured ones, it is slightly more convienient.