First off, I think you guys all recognize that the blob of troops all nations seem to build after a few years which weighs down their economy and lags the game is an issue. When Canada or the UK builds enough air units so they are using virtually every airport in sight. The screenshots where France has thousands of units filling every single hex of it's mainland.
Those are obvious issues for many reasons and there are a couple things that need to be done. Other than teaching them to reserve their units more I mean which is something that definitely needs done. One of which (scrapping units) should be a fairly simple suggestion to implement and would do a lot to fix those immediate problems and I think the second (a for sale system) would do even more, though it would require a little more work to implement.
First off. Every nation should keep track of it's defense spending and how much of it's total economy is going towards that. If at any point a nation goes into a level of debt that surpasses whatever % of their income you guys feel appropriate, then that nation should look at their defense spending and compare it to their income. If over a certain % is going towards defense spending then they should identify what their most outdated and expendable equipment is. I believe the game has a weight system which tells the ai what to build based on stats and the same thing can be used to determine what is expendable.
Now.... once it's identified what is expendable, what it does after that depends on whether or not you just teach them to scrap those units or if you teach it to put those units up for sale.
Ideally, I would like to see the nation able to flag those units as "for sale" so that all smaller nations that aren't actively trimming their defense spending as I explained above can compare the units that are for sale to the units they currently have and if the new units score higher than those they have then they will put in a bid. Out of the countries that placed a bid, whichever one has closer ties to the selling nation and a stronger diplomatic score will get the unit. There should also be a check when buying units to see if the nation is able to afford them. If the item for sale is an expensive carrier or aircraft squadron, then poor iceland really shouldn't be putting in a bid for it. Also, some units like nukes really shouldn't be put up for sale.
The way I envision this is that the ai as the US will continue to build the newest and best equipment it can field, yet as the costs for doing so continue to increase, it will begin looking at it's older models which are still vastly superior to most of the rest of the world and sale them off. Those lesser nations will in turn sale off their older equipment all the way down to the poorest and weakest of nations. When their budget gets too big and they try to sale, but no one will buy because their current equipment is better than what is being sold, then after a specific period of time, let's say a month or so, then they will simply scrap the unit altogether.
Basically, nations need to be made to have an idea of how much of their budget is going towards defense spending while keeping an eye on what it's doing to their debt. The further in debt they go, and the more their defense spending takes up of their expenses, the more units they need to shed. When they shed enough to get to manageable levels again, then they can scan the "for sale" units in the world to see if there are any they should buy until they reach their defense spending limit again and the whole process repeats.
Combine these things with an ai that reserves their forces more often and some economic improvements to the game and I think we'll have a much more enjoyable game!
Would probably help if coutnries that needed units could make offers rather than only the seller being able to offer.