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.