Terraforming
Actually, the terraforming comment was largely a joke. Personally, I think it would be easier to build large O'Neill colonies than terraform a terrestrial-sized planet.
If we can crack the problem of permanent biological adaptation to zero-G, large non-spinning space colonies become orders of magnitude easier to build, and the usable living space increases drastically. Whittling out a large asteroid could give you a lot of living space if you didn't have to worry about spinning it for gravity.