Blizzard added new Motion Sickness options for Dragonriding in Patch 10.0.5.
You can enable them in the Game Options under Accessibility > General.
Landscape Darkening applies a darkening effect around the outside of the screen during Dragonriding. The screen turns darker the faster you fly.
Focal Circle adds a circle crosshair to the center of your screen.
Both settings can be adjusted separately and enabled at the same time.