As I understand the interest rates were added to make people store money in the banks, instead of storing in tents/bases. However, there occured problem, couse with the current mechanics all rates equals 0%. I'm not sure what was developers intensions, when making the interests rates, and I dont know the whole economy mechanics, but maybe inflation system will improve it.
If developers will make money constantly inflate, that is, if developers will make all prices and wages (items base prices, and everything else, even shit shoveling) still constantly increase (let say 1% a day) and make bank's intrest rates equal inflation rate or higher (lets say depending on capital collected in bank, just like now), people will locate money in banks just to make them not loosing value. The value of 1% of the money, that is not stored in banks, could be transfered on banks accounts, to make interest rates work. It will probabily also make people store rather items, not money, so it should make selling items easier, however, real barter skills will be useful, as the prices will still change. Also it is quite realistic, I think, and it will make game economics much more complex and interesting.
If it is hard to implement, maybe it is possible to make caps stacks in inventory, tents and bases (and laying on the ground) decrease in number, or be transfered to the banks.
Probably, many people will say its stupid and nonsens, couse they like current system, but I think it will bring live to economy, make barter players more important and it will make game more difficult even in the economics. Also it will move the cash that gangs stock in their bases, couse now observed lack of cash is probably caused by that (now it is quite good sytuation with cash flow, because the wipe is near, but at usually changing items on cash is problem for not gang players).