This has probably been discussed, but here goes. I have a serious complaint about assigning production to HQ's. I discovered this in a game against the AI in which I couldn't understand how I was being outnumbered so quickly. We have a water world, and an island that is contested. I have two cities on the island (which are not ports) and it has one (which is a port). In two turns I notice that it has doubled in size and has two units filled with armored cars. OK, I discover that if you have a port on an island (and one on the home continent), you can assign production to an HQ to an HQ connected to that port even if you don't have any transports. The production goes to the HQ, and you do with it what you want. This is an extension, but more serious, of the same thing you can do on a continent where you can assign production to a city and more or less avoid issues related to transport. But, this seems to be a serious loophole. It is essentially possible to direct the production of an entire continent to an HQ on a tiny island far away without having to build the transport infrastructure to support it. It is all magically transported there. I did it myself. I edited the map, converted one city to a port (just one sea hex), and assigned production to the HQ, and magically I had 2 light tanks and 32 infantry from industry in my capitol on a continent that cannot be reached in one turn by sea movement. I did not have a single transport in my inventory. This makes supporting an invasion pretty simple. Get on the continent, get a port, and assign production to an HQ in the port. Frankly, this makes a game simulating an ocean war verge on the absurd.
Frankly, I am not too happy with the same situation by land. It is a gamey way to get around transport infrastructure and the readiness loss arising from reassigning units from different HQs.
Now, I also understand there is a problem with production and transport. Obviously, there has to be some kind of transport system in place, otherwise no game can even start. If you have no trains, how could you even get production to the supreme HQ. Chicken -- Egg. And no matter what you do, there will gamey work arounds. Perhaps rules like these could work as a start:
1- production can be assigned to any HQ that lies within the distance that can be reached by a truck in one turn. To make this simpler I would even suggest a defined radius, because the distance could be extended by roads, which makes it a bit complicated for the player to compute, remember and use.
2- production can be assigned to any HQ at the top level of the command chain provided the production center is connected by an unbroken chain of land hexes. Connection by a series of ports should not count.
An advanced alternative is to allow production outside these limits to to be performed only by using the current landcap and seacap transfer rules, which then limits later transfers within the turn.
Frankly, I consider this to be a serious "deal breaker" and should be addressed in a patch rather than ATP.