“Earth itself became a garden planet, bearing only one city worth noticing, the sleepy capitol of a galaxy. Pittsburgh valley bloomed, and rich honeymooners went there to frolic. Old bureaucrats went to Earth to die. Nobody else went there at all.” – James Blish; Earthman, Come Home.
Here’s an idea for a variant Classic Traveller campaign.
Use Supplement 3 (The Spinward Marches), but declare Porozlo to be Earth – it has the right physical characteristics – and spiritual home of the
Third Terran Imperium. The real seat of power is Mars (Rhylanor – again it has the right stats), which for astrogation purposes is one jump away, so represented as being in a different hex, even though it is in the same star system as Earth.
The Rhylanor subsector doesn’t look much like local space, so one also has to declare either a change in scale (perhaps one hex is 5 or even 10 parsecs), or say that the map of hyperspace is different from the map of real space.
The rest of the Spinward Marches and most other support materials can be used as is, but the Imperium is likely to be much smaller, perhaps only 3-4 sectors in size, and younger. The reduced age can be handled by dropping the first two Imperia and any reference to the Vilani, making the age of the Imperium (let’s say 1105 years) as far back as human interstellar travel goes.