Hrmm, replacing PDC's with some sort of ground unit that acts as a surface to space weapons battery might be interesting.
Without PDC's, combat engineers lose their purpose. Replacing them with a large unit (Battalion sized maybe) that instead has a fairly low damage weapon that can engage hostile ships in a radius around the planet (Could probably upgrade both damage and radius through a seperate line of weapons tech) could prove interesting though. Individual ground units can't be targeted by ships, meaning an attacking fleets only option is to start nuking the smeg out of the world if they want to silence the guns without a heavy ground assault. If you want to capture more than an irradiated wasteland, you'll have to commit to a planetary invasion. Otherwise, you'll be sitting back away from the planet, less you start to get chewed up by the ground based guns.
If you do stage an invasion, the same ground guns would likely prove to be an effective way to take out any assault ships landing on the planet, weakening the attacking forces. I can't think of a time I put more than one layer of armor on any of my combat drop boats, so a few guns could help the defender balance the odds in their favor. As a whole, this kind of unit would prove incredibly valuable for defensive operations, and considering the C# version is already bringing us one big game changer to ground combat, a second could be included as well.
You could probably tweak exactly how they work by changing what they're shooting for different effects on gameplay. Letting them shoot smaller missiles from the planetary stockpile would significantly buff their striking power (Let each ground unit fire X MSP per Y increments, tweaking X and Y for desired results), give fleets the opportunity to defend with PD, and put an upper cap on how long they can fend a force off by making them dependent on a consumable, but would add some more complexity to ground forces by requiring you to keep them supplied with this ordinance.
There's probably other ways to go around it, but really, I do like the idea of replacing PDC's. Although I will miss multifaction earth starts going full WW3 with nuclear missile bases.