I've had a thought about the maps you'd make for scenarios...
In normal games there's three city sizes, the one where you build the city, then you need an aqueduct, then a sewer system, at sizes 6 and then 12; for these scenarios you should set the city limits at say 2 and 12, and use rivers to represent easily habitable planets (Many M-class planets). Then you have an expensive city improvement like a bioshphere or terraforming, so that if you colonise an uninhabitable area in the game then you have to spend a lot of resources to build up the biosphere before the planet can support a large population; however if there's a river on the planet, representing the fact that it is habitable, then the population can carry on up to size 12 (or whatever) without having to build the biosphere improvement.
Idea summary:
-Change aqueducts to biosphere, increase cost, use a size limit of ~2
-Change rivers to represent habitable regions so no biosphere is required for larger populations