Why not add bonds to the game?! Granted, I just came out of a massive test on the subject, so maybe I'm biased, but I still think it would work out nicely
Heres the formula I worked out on the way over to my dorm from my class (a twenty minute walk... gah):
5000g + (a random interger in the range of +/- 1000g) + per capita income for your civ * 100 = bond rate
10% + (a random integer in the range of +/- 5%) - (number of successful bond repayments/10)% + number of bonds defaulted on *10 = Interest rate
Up to five bond sets can be issued per turn, and all must be repaid (or defaulted on) within 10 turns. A default does not mean that the civ loses the amount of the bond, just that later interest rates will be higher and the player will not be able to issue a bond for 1-5 turns. Should the player not be able to pay the interest rates, they will be offered the ability to issue another bond, or to lose units and buildings up to the amount required. No civ can have more than 12 bond sets outstanding. A specific tech and a specific civic (which cannot be changed from as long as the player has bonds outstanding) and a specific small wonder should be required.
Implications: Per capita income would greatly influence this, so the warmonger would not benefit as much as the small trading economy or suchlike. This would also give builder civs a chance against mid game grognards by allowing them to powerpump out units while financing the crusade in defense of the homeland through bonds.
At least, I think it would work like that. Hopefully it would be balanced, in that pillaging and destruction of cities would reduce the income gained on bonds, thereby weakening the ability of these financial civs to make war in the same way that pillaging and destroying the civs of a warmonger based on production would.
The idea behind the random number ranges is to prevent a player from being able to run their economy into the ground in favor of production and then use bonds to rush buy at the same time based on some known minimum value of bonds and staggered payments.
Whatcha think guys?
EDIT: Gotta make sure that there is an option to refinance bonds.