What about adding a killer explosion sound when two planets collide.
Attatch the sound to the explosion, so that if you happened to be zoomed in on it, it would be loud....and vice versa if you were zoomed out.
Maybe not on top 10 list of things to do, but it would really make a huge difference....even some ambient space sounds would be awesome.
........Whenever I use Universe sandbox, I always play my ambient space sounds MP3.
When I think of the sound, I think of how the sound is in the game Homeworld....(A great RTS Space battle game)
(Which, by the way I don't have time for anymore
)