The Rings of Earth
Theories suggest that in the future earth will be out of all estate and no more building can be done. One way we could deal with this is ocean reclamation in which we could live on floating island or boats which is completely plausible. Either way. The rings you see circling around saturn is the keyword. An annular disc is a quite solid thing which makes it possible to build on there which means we could possibly create this with the help of the moon. We take matter from the moon and with explotions it could be ejected into lunar orbit and could protect us from radiation as well.