Okay, first off, I love dragons. I play D&D and I love fantasy stories in general... but if we're going to be raising, riding, etc. I feel like this is trying to cross the horse with dinosaurs. While I don't think this is a bad thing, I think it is a complicated thing due to dinosaurs and horses being two different types of animals in the game as it stands. By this I mean that dinosaurs are a resource producer and horses are (essentially) a tool. To combine the two?
Things to consider: since this would essentially replace your horse (because I couldn't expect anyone to have access to something better than a horse using a horse), would access to this be from an upgrade to the stables? (Think, maybe, an aerie?) Or would this be an entirely separate building? If so... why would anyone keep their stables?
The next consideration is how such an upgrade would be unlocked. Can you just upgrade it like you can a shed/barn/coop? If so, what resources would be required? Would we need to find a "dragon egg" artifact much like the "dinosaur egg" artifact? Or would enough wood/stone/etc be enough? Perhaps something where you need dinosaur eggs as one of the resources needed to upgrade the stables in order to get a dragon?
There's so many ways this could be implemented, I feel like we'd have to do a lot of brainstorming to hammer things out in a way that would work with what we have already.