Engineers

The one great feature of Supreme Commander's tech levels was that higher-tech engineers could build at their own tech level and at all levels below. This significantly reduced the amount of micro-management you had to do to build the lower tech buildings.

I agree, and think it could go one step further than already exists: metal-specialized or energy-specialized construction units. An energy-specialized construction unit would be more efficient at building energy-costly buildings, and vice-versa. A typical TA game is bottlenecked on metal when energy vastly overflows capacity, so it would be nice if that extra capacity could be used more effectively to speed up certain unit or building construction.

It always bothered me that an energy-expensive unit would take forever+1 to build due to energy, and you couldn't speed things up because more engineers = more metal, which just forces more through the bottleneck.

Why do people keep saying this? The only differences between the different construction units in terms of what they could build where:
Their tier (only 2 tiers, easy to remember)
Only ships could build underwater stuff (for OBVIOUS reasons)
The fact that units could only build factories of the same type as themselves (kbots could only build kbot labs, boats could only build shipyards; how is that confusing?).

Please explain how it was hard to keep track of ANYTHING?

I'm ambivalent on that issue.
With only 2 tech-levels it'll be a lot less onerous if this isn't the case in PA.

This has never been onerous outside of 5mins with the game. Learning what builds what is a basic task that takes a moment to determine.

Exactly.

Also, I disliked SupCom's system because it rendered lower tier con units much less useful. Then again, everything in SupCom kinda focused on getting higher tier and forgetting the lower stuff.

I agree for the orbital, air, sea etc. engineers, but not necessarily for engineers with tech tiers. True, it does add some texture, but is it worth the hassle? I don't think so, the extra depth doesn't weigh up the extra complexity. I also really don't like units whose main or only difference from others within the same niche is incremental efficiency. This either makes low-level units obsolete, or, in the best case, rewards number-crunching over strategy. I feel like there has to be a better, less messy way.

So here's my suggestion: A T2 factory is simply an upgraded T1 factory, not a new building built by a separate engineer. Taking out the middle man so to speak.

100% agreed. I'm liking zero-K better the more I hear about it, have to try it out someday.

We can combine this with another idea: From other threads it seems people would like some form of upgrading your commander to keep him useful, that is not overly fiddly, P2W or a MMORPG grindfest obviously. I support this and think an elegant way to do it is if any advanced factory can upgrade your commander's chassis.

So adding that up, here's a rough outline of a way to streamline engineer variety and building tiers, while adding sensible commander upgrades.

Mech factory: scout mechs, medium mechs, walking bombs, walker engineers (standard; cheap)
-> upgrades to Advanced Mech factory: Jumpjet mechs, AA mechs, Krogoth-lites, walking nukes, Commander mecha chassis

Spiderbot factory: spiderscouts, medium spiders, spiderbombs (burrows?), spider engineers (fast; can climb anywhere; burrows?)
-> upgrades to Advanced spiderbot factory: Giant spiders, Leaping/amphibious spiders, Spidernukes, spybots?, Commander spider chassis

Wheeled factory: scout jeeps (fastest ground unit), assault trucks (fast; fairly weak), APCs, wheeled engineers (super fast; cannot climb mountains)
-> upgrades to Advanced Wheeled Factory: Uhhh, some kind of rocket car? ...Commander sportscar chassis?

Tank factory: Light tanks, Medium tanks, tank engineers (heavily armed and armored; cannot climb mountains);
-> upgrades to Advanced tank factory: Hovertanks, Heavy tanks, Monster tanks, Commander tank chassis

Artillery factory: Light artillery, medium artillery, artillery engineers?? (ranged builders???)
-> upgrades to Advanced Artillery factory: Rocket artillery, Heavy Artillery, Commander Artillery chassis (essentially mobile strategic artillery)

Shipyard: Destroyers, cruisers, transports, naval engineers
-> upgrades to Advanded Shipyard: Battleships, subs, carriers, Commander amphibious-submarine-battleship-supercarrier chassis

Airport: Fighters, bombers, Transport planes, flying engineers
-> upgrades to Advanced Airport: Strategic bombers, helicopters, Stealth bombers, Commander airplane/helicopter/starscream chassis

Spaceport: Space transports, Satelites, orbital engineers
-> upgrades to Advanced Spaceport: Advanced satelites? Mothership? Commander spaceship chassis?

The remaining issue is those special high tier buildings that are not factories, radars etc. This can be solved by letting only an upgraded commander build them. This is a very drastic change, and may be flawed. I think it could make the game more intense, however. So only an upgraded commander can build the following: Orbital Death Ray, Planetary Thrusters, Strategic Missile Silo, Interplanetary Artillery, Superweapon Factory. Hell, maybe you can drop the superweapon factory and force the com to build every superweapon manually. Then, in addition to choosing which chassis to upgrade, you need to choose whether your com should lead from the front or stay in base and build superweapons, and then which superweapons to build. The idea is not just to keep the commander relevant, but to keep him as the most useful unit. He should always be the weight that can tip the balance and the player should really chew his nails when deciding how to use him. A less restrictive way to do this is to let normal engineers build these things too but only in the vicinity of the commander. There could also be a CPU-but-no-chassis upgrade, which for a lower cost gives you the ability for advanced construction but no extra physical power.

(sorry for going OT, sometimes I can't help myself :|)