I've been reading the Saga of Seven Suns by Kevin J Anderson and there are great prossiblities for GalCiv in there.

First and foremost two new aliens, one that lives in suns and second one that lives in Gas Giants. You can imagine the new weapons and shields that will be required to deal with either one. (Nitrogen shielding, Ice weapons | Diamond hulls to survive the pressure of Gas Giants)
To limit the number of usable solar objects the sun type or mass of the gas giant could play a role here.

Secondly mining Gas Giants for fuel could be an excellent addition, great strategy could be employed.

These are just a few, there are many other great ideas that could be used from this series.
It might be difficult to balance but would certainly give a truly unique experience.