First off, I like the idea. Getting rid of PDC's streamlines a ton of stuff. If you still want silos, there are still stations. Also, changing the energy/ballistic weapons in atmosphere would be a good thing.
Lasers
can push a lot of energy through atmospheres even with degradation. In fact, the secondary thermal effects could be interesting. Kinetic weapons actually would work pretty well. At the velocities railguns are pushing, there would still be impacting with enough mass and energy to ruin somethings day. Ablative heat shielding would be pretty easy to add.
So, on vehicles. With what Steve has already said about fixing them at distinct tech levels at the time of manufacturing, this starts suggesting something like the current fighter design process. So, you could take the base line armor and adjust the amounts (increase/decrease mass), slap on a weapons system and have a basic configuration. Additionally, you could arbitrarily set a threshold for design mass to dictate the vehicle class similar to fighter and gunboats. So, less than 50 tons is a light armor vehicle, more than that would be a heavy vehicle. For that matter, you could tie that into the planned Titan system as well. Less than 50 tons, light vehicle, greater than 50 is heavy, and greater than 200 (for example) gets into Titan country. They still use the same design process, like fighters/gunboats/ships, but can be handled differently.
So, that would give you a basic vehicle class, basic armor value (defense) and a weapons system (offense). In the interest of keeping planetary combat somewhat abstract, you could then modify your new system using initiative to dictate combat order and modify that by vehicle class for example. You could potentially modify that by adding additional systems to improve that initiative value (via command and control systems or sensors), or modules like troop transport bays that add to initiative for infantry in the region, or logistics modules to boost supply. Not to mention modules like ECM and ECCM which would be critical in that kind of high tech battlefields. All those other modules add to mass and help dictate class.
Infantry could be handled the same way. We already have leg infantry, and powered armor infantry, so you could do the same thing with them.
Leg infantry: Cheap, low init, low armor. This would be your basis for things like Garrison units. Cheap, lightly equipped occupation troops. They could also be planetary militia. They might have some powered suits for a rapid reaction force (SWAT units for Garrison for example) but the bulk is cheap leg infantry.
Assisted Infantry: Mixed units of leg and suits, or troops with power assisted armor. The suits provide some assist to load bearing (better init) but may not be fully sealed for things like vacuum or hostile environments. Or if they are sealed suits, they may not have the defensive systems on full power suits.
Powered Infantry;There are the classic Starship Troopers type power armor (the book, not the movie). Heavily armored, high load bearing, high mobility. And expensive as hell.
Now, you can add weapon systems. Like a basic small arms package like;
Gauss: Low damage, high rate of fire (init bonus)
Rail: Medium damage, medium ROF
Lasers: High damage, low ROF (init penalty)
If you wanted to complicate infantry a bit further, you could give them support weapons (as an option) to increase their offensive (damage) potential at the cost of initiative (more heavy stuff to drag around) to give them more staying power or anti-armor potential.
So, for example:
Garrison Infantry Company, Armor 1, Damage 1, Init -10% (Leg Inf -20%, Gauss Rifles +10%) Training Cost 20 Duranium 15
Garrison Infantry Battalion, Armor 4, Damage 1, Init -30 (Leg Inf -20%, Gauss Rifles +10%, Battalion -20%) Training Cost 60 Duranium 60
Heavy Infantry Battalion, Armor 8, Damage 4, Init (Assit Inf 0%, Rail Rifles 0%, Anti-Tank Missiles -20%, Battalion -20%) Training Cost 180 Duranium 300 Neutronium 40
Marine Raider
Finally, you can take all your ship sized weapon systems and slap them on the Titan/Bolo/Ogre/CSU/Helltank platform. They would be big, and baseline init would be low, but you would have your mobile Anti-Orbital battery, or your Warlord Titan build to smash ground vehicles.
Anyway, just my initial thoughts reading this far far to late in the evening.
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