i believe the new eeveelutions will be based off flying (winds/high) and ground (waves/low)

so heres my reasoning why, the painting that shows the two eevee's demonstrates a field where you can see the sky and the land. Alot of people couldnt figure out the waves part but i dont think it had to with water but more high/low altitudes which is why flying and ground are the twins for this upcoming gen