If the contintent itself is designed with flying always being present and where flying is a mandatory part, I don't see any problem. Sure, if they design the continent the same they do with the others, flying is just the convenience. But if your vision for Dragon Isles incorporates the gameplay attached to flying, it's something different.
This could easily be down by e.g. giving the player the moment he steps onto Dragon Isles a dragon companion / mount. You can only use that to fly there and it's slow. You need to level up your companion and you can even customize it later on (which Dragonflight its from, gender, flying style etc.). Flying isn't unlocked from the go, your working towards it in every zone with your dragon companion. You start with slow flying (slower than the fastes ground mount speed) and can slowly upgrade it if you fulfill different tasks etc. It's really easy to give players a new flying experience (at least for Dragon Isles) that would work and be incorporated in the game mechanic. Heck, your Dragon companion could be one of the new gameplay features for the expansion (that's cosmetic; character progression is another system).