Now, this has probobly been suggested before, but since I can't seem to find anything of the sort in the archives I'm going to suggest it anyway.

How about having stuff orbit the stars within the systems instead of staying in one place all the time. The contents of one sector would move to an adjacent one every x-number of days, and each orbital 'ring' in a system would have different shifting periods (ie stuff at the center would cycle quickly, but stuff at the edge of the system would rarely move, if ever). This sort of stuff would require people to periodically change their navroutes, and make it slightly more difficult to keep track of uncharted asteroid fields (among other things).

I admit that this would probobly be rather difficult to implement, but hey--I can dream can't I?