I've always toyed with the idea of getting FG to explore the atmosphere of a gas giant - all clouds, and none of that nasty terrain
You'd have to modify the atmosphere definition(s) - there appear to be several, for instance JSBSim brings its own atmosphere code, but then there is also a FG generic, and what model is used depends on the FDM. I'd look in the source code for any files containing 'atmosphere' in their name - that's probably a tell-tale... The same goes for the gravitational constant - presumably that's inside the FDM code.
The appearance of the atmosphere is computed inside the skydome shader - so the new parameters would have to be fed to that (I think the scaling altitude is hard-coded and not a parameter, but atmosphere density and density of Mie and Rayleigh scatterers can be modified runtime from the GUI, so it's not a problem making the atmosphere
look much denser.
So it's really some not so extensive set of changes.