My suggestion: Allow tribes to have 1 (one) tribal area that can be designated safe. In order to keep safety they will need to pay tribute or an upkeep. These safe cities can only be created within certain, connected, zones that the developers designated. Within this area the only land that can be built upon and terraformed would be within tribal lands.
Outside of this designated area, players will be allowed to build and terraform as they choose. To make this balanced, buildings should have obscene amounts of hitpoints and/or resistances. Terraforming on previously un-terraformed land, or land that was returned to its pre-terraformed state, will be like it is now; however, each additional type of terraforming (even if it's to undo the terraforming) would become more and more difficult, and require more and more resources.
The lore considerations for this are thus: The "forces of good" have been able to drive back the "forces of evil" and have granted their protection over land. In order to keep the "forces of good" powerful and able to protect an area tribute must be paid to them.
Edit: Idea: There could be buildings availible for tribes to build where they can pay tribute for certain buffs on their buildings outside of their safe zones, and, perhaps like in RTS games where one type of resource could be paid to increase members harvesting rate, or decrease the amount of items they deplete from a resource node of a different type.