So in my mind this feature works as follows:
The world's 5 major weapons manufacturers are typically referred to as being
USA
UK
France
China
Russia
Russia has dropped down, but IMO is important enough to be kept on the list. Post WWII these are the big 5.
Here is the
wikipedia page to use as a reference.
So these are represented in our game by the following civs:
USA
NATO
EU
China
Russia
The WEAPONS LICENSES will function like resources. Ideally traded from a separate drop down list on the diplomacy screen.
Each of these 5 nations will have (at least) one custom unit for each class. By default, they are the only nation who can build their units.
Without licenses, any other country in our game will only be able to build very rudimentary units, plus their special unit(s). We have to decide whether or not we want to give every country one set of licenses they can lease for their special unit, or just keep it to the big 5. This I think would depend on how much turn time this feature eats up.
So, the big 5 companies have a special building for each unit (Lockheed martin headquarters, etc etc, we'll have to get creative, maybe some buildings give multiple licenses). Each building gives the civ one license (resource) for every civ in the game (so 48), for the unit(s) it represents. These licenses can then be traded (leased) to others civs who have the required technology and resources, which enables the purchasing civ to build the unit in cities with applicable barracks/air force factories/dockyards etc.
Gaining one of these licenses can sometimes reveal techs. Some of the more advanced ones will probably have to be quite expensive with the AI very unlikely to trade them at all. But ones like say Russia's T-72 will be cheaper and quite likely to be traded for a fair price to all but enemies of Russia.
So a quick example;
You are playing as America. You start with a Northrop Grumman headquarters in Washington. This building gives you (the USA) 48 licenses for the "B-2 Spirit" unit. So immediately, you can build it. This works like how getting oil lets you build units, only you are the only person who can get this resource (license), like how Broadway gives you the Hit Musicals resource.
You then have the option to rent out licenses to your friends and allies, exactly as you would resources. This example is one that only the USA has right now, so at least until a civ catches up to the related techs that the US will exclusively start with (stealth, GPS for example), there should be almost no circumstance of them leasing it. Even once other civs catch up, they should only lease it to their closest allies.
So, repeat that for every class of unit for the 5 big arms dealers, and boom, we have a much more realistic representation of how militaries are built, and the profits that go to the dealer nations. Not to mention, each country's army will remain unique as they expand, since different countries will have different combinations of licenses.
We may want to make it 6 and give Japan a bunch of licenses, or 7 and include Saudi Arabia, or 8 and include India. Maybe these nations should simply have fewer licenses than the big 5. Perhaps sheep21 can enlighten me as to what would be appropriate here, he clearly is the most familiar with what is in production and by whom.
To avoid making this an AI/newb trap, we will have to limit each non-dealer civ to being able to lease only one license per class at a time.
Ideally of course, it would be great if the AI could focus first on acquiring licenses for unit classes it needs. I think it may work best if a civ has to have (X) amount of oil saved up before being able to buy licenses for units that require oil (to avoid them spending all their money on licenses while they can't make units), but we'll have to tinker around with that when we see how the AI handles it. Perhaps they only try to acquire new licenses at times when they want to build military units, and only keep a few cheaper licenses in peace time.