I have been playing US2 for a while now, and I'm not sure if it's in the simulation, but can Deuterium Fusion be implemented as far as solar compostion? If you're not familiar with Deuterium Fusion it occurs with masses between about 13 and 80 times the mass of Jupiter and respectively form Brown Dwarfs with a lifespan of ∼4–50 Myr.
I recently tried to 'ignite' a 14 Mass Jupiter, and although the temp rose, it started decreasing
I'm sure they've been discussed as well, but Neutron Stars would be a nice addition also
Here's a link also for further explanation:
http://en.wikipedia.org/wiki/Deuterium_burning