Make flying PCs and flying polymorph shapes immune to spells that involve walking on the ground.
Flying PC shapes:
- Pixie (race and Polymorph Self shape)
- Imp
- Wyrmlings
- Parrot, eagle, raven, and bat totems
Ground effects:
- Grease
- Earthquake
- Entangle
- Spike growth
- Vine mine Entangle and Hamper movement
In return make them weak to gust of wind, web (I imagine it's hard to fly around the strands created), and other spells that make sense to be more challenging to a flying creature than a walking one.
Also allow flying forms to auto-succeed climb checks and travel via lasso spots freely.