An interesting idea came up in EFU Discord!
Replacing the Natural Swiftness feature, this would allow a Ranger to choose their Favored Terrain in addition to their Favored Enemy. The perk would be limited to pure rangers only. The effects of the perk would only be applicable while on the relevant terrain.
There are three obvious options that fit within the existing zone archetypes.
1) Wilderness (+1 Reflex, +10% movement speed, +2 hide, +2 move silently)
2) Urban (+1 Fortitude, 5% slashing immunity, 5% piercing immunity, 5% bludgeoning immunity)
3) Planar (+1 Will, 10% elemental resistance, 10% negative resistance, 10% divine resistance)