Meh, go back to the old terraforming.
Create a new class of siege bots that can tunnel through terraformed ground quickly and destroy any objects or structures placed on that ground. Think some new bot that when an active module is run will flatten terraformed land to a flat plain without targeting anything, just as an aoe aura...done quickly to negate 15hr terraform sieges.
Then create a buildable wall structure system. My thoughts involve wall turret structures with a ramp for light mechs to climb, must be placed XXX distance from other wall turrets, when powered an energy shield forms between the wall pieces creating a wall that cant be sieged from above mentioned digger bots.
With this you have the ability to get creative with terraforming, the means to quickly dig through it via siege bots and wreck placed structures on the terraformed ground, as well as a means to build legit and decent looking walls with functional ramps for defense.
IMO this is better than taking away content (terraforming plans) as a means to negate the OP nature of terraforming, which is just going to be exploited in another manner. At least if the above mentioned stuff can be exploited we have content, more bots, and variety to play with..
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