I myself have no issue getting rid of PDC's. I always seem to have problems with them and as long as something else provides the basic functionality, then that works for me.
One idea, getting back to troop types. I see Steve is looking at introducing predominant terrain, which introduces new potential units:
airmobile units, heavier lift requirement, higher firepower, lower defense, ignore terrain (basically like air car air assault infantry)
light infantry, large bonus in light terrain, low firepower, low defense, low lift
normal infantry, normal bonuses
armor, heavy lift, high offense and defense, bonus in open terrain, penalty in rough terrain, x2 reinforcement cost to heal
field artillery, no direct attack, just get a moderate Titan style bombardment attack
air defense artillery - CIWS feature
I would do missile batteries and/or energy batteries as installations
Also consider a Special Forces unit, which would be expensive to train. They would not engage in standard combat but can be used as follows:
* Advanced force operations/pathfinder - increase off load and drop times for conventional forces if already present
* Infrastructure attacks, random chance to bombard enemy infrastructure/installations
* Ability to bombard air defenses and batteries if possible along with deep space sensors
* Psyops, randomly attack morale of enemy units on planet
* Sabotage construction on planet
* In cases of reconquest, train local light infantry to rise up
* Recon, Surveillance and Target Acquisition (RSTA) - increased bombardment percentage from space ships when SF present
* Provide a substantial bonus to help pacify occupied planets, basically targeting insurgent cells (love the F3EAD!)
I think they would be autonomous, in that the above effects would just be random every cycle or automatic in some cases (like RSTA or Pathfinder).
Add some kind of long range insertion technology to drop the SF teams.
Add mechanic so that enemy forces get a chance to find and engage the SF teams each turn unless conventional forces are present, which should have a curve that makes it dangerous to stay too long without conventional forces present
Have a counter insurgancy