Will the way ship movement is calculated be more advanced when travelling between two moving objects (like planets)? I.E. will the ship take its travel time into account and move towards the expected target position at that time, or simply calc its movement to the actual position of the target and then every movement cycle "adjust" to the "new" position?
Fleets will adjust their course to intercept moving fleets but not system bodies. The problem in the latter case is that the body could be a moon orbiting a planet orbiting a star which is orbiting another star, which itself is orbiting another star (and everything is moving in an arc, not a straight line). Trying to calculate the best intercept position for a fleet and a system body in that situation is not straightforward. Doing that for a lot of fleets would be a lot of calculations.
As system bodies move slowly compared to fleets, I think the performance downside of all those extra calculations is probably not worth the game play benefit.