Terraforming Candidates w/o Atmosphere
Terraforming a planet involves deleting a planet's native atmosphere and replacing it with an Earth-like one. In that sense, it's probably easier to do this on a planet that has no native atmosphere to begin with. Creating a new atmosphere
ex nihilo is an engineering problem, not a laws-of-physics problem. They've got machines and/or microbes to break down carbonate rock to release CO2, and convert that CO2 into biomass and oxygen. Nitrogen is trickier but finding a mostly-nitrogen comet and towing it into a decaying orbit around the planet would be a logical solution, though you might need some magical hyperscience to convert any ammonia into nitrogen gas. Finally, they could rely on players to ship in the required atmosphere, at a few dozen to a couple hundred tones per shipment. Earth has about 5.5 quadrillion tonnes of atmosphere, so it might take a while to do it that way if you're starting from scratch, but terraformers don't seem to be in too much of a hurry to get the job done.
A harder planet to terraform would be one with a surplus amount of gas - for a planet with 10 atmospheres worth of nitrogen on it, for example, you'd need to get rid of at least six of those atmospheres - and getting that much gas off a planet is much harder than putting it there in the first place. You'd need to liquefy it, then either store it in vast reservoirs underground (and hope it never leaks out), or figure out some way to solidify nitrogen in a non-toxic fashion, or ship it off-planet (again, using players buying it in one-tonne canisters). But again, all of these are engineering solutions, and not specifically prohibited by the laws of physics.
