At game setup, add a new menu with "special options", each with overrides that change galaxy creation to force a different style of game play.
Such as,
:Stella Rasa:
"The stars are empty. The Hyperspace Collapse may have been something far more than understood."
At galaxy generation, the Stella Rasa bit skips generation of minor races, refugees, pirates, and marauders. The stars are silent and empty of all life except the Star Harpies and the starting empires.
(This would allow for a simple and direct style of game play.)
:The Birds of Carrion:
"The stars are thick with those that feed on the war dead."
At galaxy generation, the frequency of Star Harpies is doubled, making expansion a constant fight against them. Conversely, the wreckage located / orbital debris specials occur twice as often as well.
(Combat becomes important early, else the player must find paths between infested systems.)
:Titan Steps:
"When all the worlds are massive, every gain or loss is amplified.'
At galaxy generation, this overrides planet size. The distribution for all worlds is 20% Huge, 50% Large, and 30% Medium. Small and Tiny worlds are not generated.
(Because every planet can be an individual powerhouse, every gain or loss in expansion or war becomes significant. It may be easiest to have this ignore what is generated by forced_planets_fun and epic_systems, allowing them to make exceptions.)
:A Journey's Thousand Steps:
"When all worlds are small, management becomes critical."
At galaxy generation, this overrides planet size. The distribution for all worlds is 30% Medium, 50% Small, and 20% Tiny. Large terrestrial worlds are not generated. The exceptions are gas giants, planets generated in forced_planets and epic_systems, and also all empire home worlds have their size set to Huge. Planetary Construction is not affected.
(Expansion becomes very slow and deliberate, while the home world is forced to become the major powerhouse of the empire. The player is forced to optimize many small worlds for growth. Enemy home worlds become major targets, as do worlds like Gaia and Waterless. Planetary Construction becomes a critical late game tech, as it can create large worlds. Marauder worlds would be set to medium, making them valuable targets.)
I'm sure other options could be created.
Some would have to be mutually exclusive, like Titan Steps and Thousand Steps above. Some would mix well. Stella Rasa and Birds of Carrion together would be a very bleak, empty graveyard of a galaxy.