I do not know if this has been suggested before, but I would guess not, as the current wall decay mechanics are quite new, and I skimmed the last couple of pages.
The suggestion is just a small change to the current terrain-spreading rule of trees, which I think could have interesting consequences.
Make it so that trees growing has a small chance of turning paved/bricked/gold laid tiles to their native terrain.
Currently (I am pretty sure), paved tiles are immune to trees spreading their terrain.
Also make it so that tiles under buildings and walls can be affected.
I do not know how the terrain spread is decided now, but both paved tiles and tiles with buildings could have stacking "defence bonuses" against terrain spread, if needed.
This suggestion assumes that the current wall decay mechanism, or something similar stays in place.
The Ents' Marching Song
We come, we come with roll of drum: ta-runda runda runda rom!
We come, we come with horn and drum: ta-runa runa runa rom!
To Isengard! Though Isengard be ringed and barred with doors of stone;
Though Isengard be strong and hard, as cold as stone and bare as bone,
We go, we go, we go to war, to hew the stone and break the door;
For bole and bough are burning now, the furnace roars - we go to war!
To land of gloom with tramp of doom, with roll of drum, we come, we come;
To Isengard with doom we come!
With doom we come, with doom we come!