How about most planets will be barren/toxic/uninhabitable and you'd have to place biospheres, re-stabilize its magnetic force-field, and make transfer harmful materials?
Lets say you landed on a harmful planet but full of materials. Your AI suggests that the soil is ready for growing planets and rich in nutrients. You place a biosphere and create a magnetic forcefield that blocks solarflares from damaging the plants inside. After you see that the planets are growing giantly, you call your alliance to suggest terraforming this planet. The alliance agrees and starts funding materials so the planet will be a giant "garden world". After a while, a rival alliance sees this planet and see show much you are benefitting from it, so they send a bunch of "dirty bombs" to "de-terraform" it.
Plez, NQ.
Moar Ideas:
You just found a planet FULL of life, but all the plants/creatures are harmful to humans. You'd have to "de-terraform the planet, then re-terraform the planet". After being de-terraform, the planet loses stability after a few years without life. The planet becomes to unstable the planet basically implodes (because... that'd be cool). All constructs within about 1000km are heavily damaged. The resources will then recollect later to recreate the planet, albeit different.