What about adding one or two dwarf planets (Maybe Pluto and Ceres) ? But placing them farther away so they are not visible from the other planets?
I was planning on adding Pluto in there, it would be a good idea for them to be not visible though, it gives players something to look for.
Reactor Worker or someone made a kill Trigger if you were in the neighborhood of the Sun. You should do the same by the gas giants.
I'm actually planning on making the gas giants habitable, with floating rocks inside for building on, but the inside would have no gravity or very, very strong gravity to make building a pain in the ass.