Orbital Mechanics would be cool to...
Such that everything placed in the gravity well orbited the planet at different rates depending on the distance form the planet.
You would have to time when you wanted to attack, (when most of the defences are on the other side of the planet

).
Ships would have to be parked outside the gravity well to remain stationary.
On a larger scale, the planets could orbit the star, and you could have reconfigureable phase drive lanes.
i.e. A phase drive lane can only be so long before it reconnects to the next nearest planet. Ships in the lane when it switches could be lost... or make it to the new destination.