One of the things I wanted to change while I was overhauling the economy for this mod, was the inability to have negative gold reserves. Real nations can and are heavily in debt, and this should be allowed in Civilization as well.
However, I'm not sure how to create a system that will allow debt, but discourage its abuse. (E.g Wartime debts are acceptable, peacetime or long term debt is not).
Maybe allow loan system in diplomacy, which would look like following - borrowing certain sum for certain amount of turns (5, 10 - ?) with interest in % agreed upon both sides, and penalty % per turn when debt can't be paid in time (optionally). Maybe make it so that loan amount (total loan from multiple sources) cannot be higher than amount GNP x2-3-4?
When agreement is made, there will be agreed period during which loan taker should accumulate money, after which money/science slider moves to gold and gold automatically substracts to creditor (this can be done for few turns till payment and % are fully paid out). Also % per turn in case of delaying debt should be added too. Declaring war upon creditor should not effect loan.
Also, as an option, units and cities (we can trade them now anyway) could be made as objects of safety deposits for loan (as alternative to % penalties if not paid in time), so creditor can hold them to himself if loan isnt paid in time.
p.s. Btw, why AI never trades their own cities tho sometimes they are red in menu and sometimes white, and neither it gives anything for player cities? not even a 10 gold. While they often request them as tribute.
[edit] So for example, i decide to borrow 1000 gold from Gilgamesh, default time is 10 turns, with 10% interest per turn from base amount. Near gold slider i will have string (1100 debt to gilgamesh, 100 per turn, 9 turns remaining). At any turn after that i can contact him and (if i have money) payout debt and %s (counted for turns passed). After 10 turns pass, there will be 5 turns frame during which %s increase to 15%, and if i don't pay by that time, all sliders go to gold, % move to 20, and it starts automatically paying gold to him until everything is paid (btw this can have severe effect on monarchy where you get
per 15% tax). % can be fixed amount or vary depending how much gold OTHER players have available (just like with resources relative trading value) and depending on civ attitude toward you.
Also it should give minor diplomacy bonuses or penalties ("You loaned us money" +, "You paid your debt in time" +, "You refused to loan us money" -, "You are an untrustworthy person who doesn't pay his debts"-).
Also i think if you declare war on person you gave loan to debt should be cancelled, however if i loan money from someone then declare war on him, debt stays and works as usual.
And finally! Capitulation works now so you only can demand actual money (or money per turn) defeated civ has, with this mechanism it would become possible to request contribution (with low %'s) which would be paid in nearby time. Also, units like money and cities should be terms of capitulation (currently they aren't).