User blog comment:Chaosanc/Thoughts and Revisions to the ME3 ending/@comment-5003949-20120415154231

Using the wreckage of the crucible, citadel, and the parts of both reaper and the allied fleets, they could build a mega ship with the capabilities to reach the home worlds of the different races (humans would have to settle a new colony) spend the time dropping everyone off, everyone spends some time making their own FTL cores, and use the previously mentioned mega ship as a new faux citadel.

Granted thats only for destroying the reapers, i guess in the other options you could just ask the reapers for a ride home.