[Astromancer]

Try this idea. Alien contact in the 1940s, if you like, at Roswell. The Aliens find humans advanced enough to be worthy of rescue. In the year 2025 a wave of radiation from a botched stellar engineering project will rip through the solar system. Nothing that lacks fifty miles of shielding will survive.

The Aliens offer Humanity a lift.

It's now the year 2347. The radiation has subsided and Earth can be resettled. Some terraformation will be needed. Different groups that left Earth feel they should either get first pick of the land, or simply they alone are worthy of owning the Earth.

Basically a reworking of GURPS:Alpha Centauri. Different cultural groups fighting for dominance politically and memetically.

The simple fact of off-world backing and aid both limits and extends what PCs and their cultures can do.

Many things that made no sense in a low population frontier area cut off from the outside world can now be made logical/functional.