I agree chronobomb.
Making planets larger, and at the same time dividing them up into regions that could be programmed to have an orbital assault aspect to the game would be very interesting.
In this way movement and combat in 3d space would be made relevant, as you would attack the most vital aspects of a planets surface.
Naturally i'm implying that the developers would have to improve the mechanic of planetary acquisition to include using a form of randomized planetary combat. I'm not suggesting 3d ground battles, but some form of planetary territorial conquest would be VERY engaging and add one more layer to the fight for an important planet.
This would also make Z axis control far more interesting, as important locations on a planet are fixed and would require a fleet to perch above them to begin the space to planetary "amphibious" assault.
This would also make planetary bombing far more interesting, as you would be actually bombing actual locations on the planet instead of having the planet rotate and you bomb the equator out.