Gas giants with habitable satelites.

It seems like the easiest way of accomplishing this is to just increase the chances that lunar anomalies spawn on gas giants, thus providing moons and population slots in one fell swoop.

The alternative, which is what you seem to be going for, is to have moons that are themselves (tiny?) planets, linked in some way to their parent gas giants, and potentially increasing the number of planets in-system to more than the default maximum of five. I like the idea, but wonder about its feasibility, because it seems a bit recursive in ways ES2 doesn't really go for.