Are Terrain auras currently something which is being thought about? For example, snow or dense forests negatively affecting walk speed, paved tiles or paths positively effecting walk speed? I know Delenda Est uses, what is kind of a dirty hack, by adding (invisible)markers to farmland in the map data, and those markers have an aura.
Something I want to do is make constructable roads and pavements, and give those a speed bonus. I believe the guy making the City Building Mod wants to do the same thing and so just gives the CC a movespeed aura to simulate the effect of a paved area around the CC.
Some uses I can think of.
Making Cavalry charges or chariots hard to drive in a forest
Lowering movement speed in a forest
Roads and tiles with movespeed bonus
Lava with a negative HP aura (very hot)
Snow with a negative HP aura (very cold)
Spread increase or armor bonus for forest, to simulate trees being used as protective barriers. This might make Javelinists more effective than archers in Forest, which might be the reason why the Celts never really used archery.
Britons getting a stealth bonus when in a swamp, as Cassius Dio claims they can live in a swamp for days with only their head above water