Think about your future Earth from the perspective of the colonists. Did Earth try to rule its colonies? Or was it just their common point of origin? If the former, what was the nature of that rule? Totalitarian? Laizze faire? Something in between? Then let your answers to these questions suggest an appropriate name . . . which might well vary from colony to colony.

For example, on a colony world where the colonists were transported prisoners, Old Earth might be known as something that translates into 21st English as "Home Of Bastards." On a world where the colonists were self-exiled religious extremists, OId Earth might be "Satan's Throne." On a world settled by the wealthy one percent who could afford to terraform their new home into a paradise, Old Earth might be "that s**thole."

[That last one was stolen from Ron Perlman's line in Alien: Resurrection.]