I'm glad others have thought of this, from placing a Build Block on my planet and seeing it present statistics (for the plate I believe, since its 287x70x274?) I immediately thought of planetary systems, first thinking of shields but also could be other things like extractors or repair bays, but that's all for other threads.
I very much like the idea of being able to protect my precious rock from being taken out from space, and i think it would provide a crucial element to force extra creativity in vehicle design and ground combat tactics. Side tangent, are there jet packs for personal use? (personal size stop computer basically) I personally love the mining and ship building part of Starmade, but making ground combat necessary would in my opinion add a lot of depth to the game.
I like all the ideas in this thread so far, having the shield computer/block bore a hole to the stars to force the defenders to leave an entry point. (sci fi question, how does an astronaut survive jumping through a hole that destroys the steel and rock we wanted to cover it with?)
Having planetary shields be cheap to make so as to overpower ship weapon systems of similar investment I think is important, otherwise the mechanic has little benefit and won't force other solutions than bombardment.
Perhaps make it so that some kind of Ion weapons can pass through a shield to temporarily disable ground batteries, that way having ships in orbit outside of the shield is still important? Make it so that they don't disable the planetary shield computer, cause that would just be lame and boring. They would need to be of a certain strength to even pass through, based on planet shield strength, to encourage battleship designs for bombardment? Also possibly include Ion bombs on whatever bombers you send thru the atmosphere for the same effect, although that depends on how hard ground defenses are to take out with conventional weapons on those same fighters/corvettes/bombers. Hmmm.... may reduce having the shield's benefit too much, might be better to just need to use bombardment to weaken a spot for dropships to pass through. Idea needs more thought.
Regardless,
super +1 to planetary shields being a thing