Alot of you probably have seen that you can set up some decent loan margins by exchanging lump sums for payments over 10 turns. I think it'd be cool if this was expanded (if just superficially) to an UI option that sets an If:"lump exchanged per turn" or "per turn exchanged for lump" then an additional output box would show the percentage of the interest.
I'd have to admit it'd be dreaming to see it an official diplomatic treaty/option. The option provides the typical input boxes for the lump (to be exchanged) and and an introduction of a second output tagged along for the interest percentage and the hovering over/context box would explain that diplomatic penalties occur along with the debt of the party that comes up short during the 10 turn agreement.
I'd be interested to hear what were some big returns you've made and the context that you think made it possible/the outlier! Any experiences?