I haven't found any suggestions on this subject, which is why I dare to suggest it myself :
How about selling weapons ? During my late games, I often end up giving away sometimes massive amounts of weaponry to minor civs that aren't doing too well, or are threatened by total defeat. How about creating an extra incentive, maybe by allowing civs to sell specific types of weapons (missiles, naval units, planes and tanks for example) to less powerful civs once Corporation combined with other industrial/modern techs are discovered ?
A civ would need a certain amount of army units before extras can be sold, this would guarantee stronger civs would sell them to weaker civs. Maybe also determine an amount above which no foreign weapons can be bought.
This would suggest the weaponry would feature in the trading panel once the seller civ activates a "sell" option for its unit and stack it in the capital. Weapons trade would also benefit the relations between civs. I know that a "we are grateful for the ressources you've supplied us with" already exists but its not that determining. Perhaps selling weapons would be a way towards reconciliation, allowing open borders agreements, etc.
...or, stuff countries like Iraq with Scuds for a decade, and then wipe them out with Patriots ?
Most of all a seller would not need to have open borders agreements to sell its weapons in the first place, as its necessary to donate them up to now. May I remind Europe and US secretly sold weapons to Khomeini's Iran in the 1980s, whilst selling weapons to Hussein's Iraq, at a lower price of course for Iraq. Translated in DoC terms, Europe and the US did NOT have open borders with Iran at that time...
Selling weapons to subordinate dictatorships has been a major feature of Western and Russian economic and political control over the world since 1945 and the illusory move towards "decolonization"...Iran/Iraq in the 1980s, and of course all Cold war conflicts are just a few examples.
I hate it as much as I hate slavery, but since we can now play a more historically accurate game by stuffing our civs, one has to admit, with the blood and bones and tears of human beings, I guess it would be even more historically accurate to have these little nations destroy one another with the weapons sold by the world's leading nations and why not, make them profit from it...its disgusting but hey, DoC is about replaying world history and why not, play it better, with ethical choices made available throughout the game.