I didn't find anything in the threads about this. What about having moving bodies that you can mine such as comets?

Also what about having gaseous planets that you can fly through/mine/fight in. Perhaps only special ships can operate in them due to the increased pressure.

I've thought of comets/meteors before, but wasn't sure how often they should appear, or how fast they would travel.

The Planets is a bad idea - unless the Devs spent ages on gravity physics it would be too unrealistic. Plus even gas planets tend to be solid in the centre due to the immense pressure.