It’s already there. You can fly your dragon any place that you can do regular flying.
I find it is quite helpful. While leveling in Wrath Classic some times I want to know what’s on top of a hill or around a corner so I log into Retail and send a level 70 'toon up to look around. Of course some things have changed because of the trouble Deathwing caused but others have not.
Anyway having both regular flying and dragon flying is great. If I have to cover great distances I use my dragon, for short stuff where I want to look around I use regular flying.
In general, dragon flying is like flying a jet and regular is like flying a helicopter. They each have their advantages.