The Heavensward hint could also be less looking back and more paying attention to what's happening now, the restoration is kind of carrying on the Heavensward story ark and it's very focused on the Dragons once again being willing to mingle with man, so I could very much see them coming back into the story come 6.0.
Even with Doma, Ala mhigo and Ishgard in the alliance the empire is no push over, but a united Ishgard/Dragon force would be very powerful, wouldn't be the first time man and Dragon were at war before Dragon allowed Man to ride into battle apone them.
The Dragoons, trained to slay dragons become Dragon Riders, ruling the skies and leaping from their backs in combo of spear and flame... suddenly I want this as a class...