Something i really like to see are debts. Until now you can only spend the money you actually earn. But how would gameplay change if you could take on debts?

Of course, the amount of debts should be limited by your average income. So, lets say with 40% income you get 140 Gold/turn. Then you would be able to take a loan of about 1500 Gold ... Of course, the interest would be quite high. So civs could wage war, even if they hadnt enough money right now. On the other hand, there future goals and abilities would be limited. For example, you would be hindered to lower your gold/turn under a certain limit or would be forced to build countermeasures such as markets or even dissolve your army.
Nice scenarios could be modelled around this: Great Britain being in heavy debt at around 1760 had to raise taxes, thus annoying the colonies (with the new feature from BTS!). Or a revolution/unhappiness from your people could rise, so that eventually you had to change civis to representation ... and so on.
With Representation, certain restriction would be made binding .... Of course, debt making should be tied to certain techs, like literacy or banking.

Could this even be modded into present RFC??

In Civ III you could take loans from other civs but the AI rarely granted them, and you would rarely want to grant them to the AI.

It was also very hard to negociate fair loans. The AI always wanted you to pay a high interest rate but when they asked you for a loan it was hard to get them to do so. For example something along the lines of you getting a loan of 200 gold but having to pay 15 gold per turn for 20 turns to get that. Meanwhile when you are for 320 gold, they don't want to pay more then 18 gold per turn. That's only a 40 gold profit while the AI got a 100 gold profit from a lower loan.

It would be nice to have them back though. There were some cases where I needed money right there and then and the AI surprisingly helped me out and other cases where I was able to make a decent profit. It's one of those small features that went away to clear room for other things.

Actually I intended not taking loans from other civ, but instead from your own citizens. Like Charles V taking them from Jacob Fugger or current states give our public bonds to their citizens. That way if wouldnt be something with diplomacy...

Oh, I see now.

Interesting idea. Not sure how it will work. I always assumed the money in the game represented all the money your civ had including both private in addition to public.