I like the idea of a planet strengthening mod. We had a long thread discussing it a few months ago, but I can't seem to find it now.
I did some tests back then and had some ideas. Weapon mounts for weapon platforms are good for direct fire weapons, but if you want to extend the range of missles you'll need to create new components. You can make weap platform only missles that have 20 range. Those will reach out and touch you.
I was going to make some that were smaller than typical ship CSM's. My thought was that the weapon platform was a representation of a command and control facility for a field of missle silos and that the platform itself didn't have to actually hold the missles. Of course there is no SE4 way to represent this, but by making the mssiles take up less space on the platform you can cram more in and get a similer effect.
You could also make them cheaper to build and faster. Construction on the planet wouldn't be limited by size constraints like you would be on a ship, so you could put more thrusters in a ground based missle (faster) and easier acces to construction materials shold make them cheaper to build.
Doing this you could have a planet protected by a fearsome number of missles that had much longer range than most if not all ship borne weapons. A large enough fleet and enough point defense could over come it, but it would take a lot, and they would take a pounding on the way in.
A half a dozen light cruisers should be able to destroy a border colony, but not a homeworld.
Geoschmo