So first off, this has nothing to do with economy plugins. I have a server, and seeing as I dislike economy plugins, the economy runs on gold (you can buy and sell with it). My idea is to have a plugin similar to LWC, in which you define a chest using a command, but instead of it protecting the chest, it would go through all registered chests at intervals defined in the config, check how much gold they contain, then modify their value, effectively providing users with interest. I'd make the plugin myself, but I'm new to all this, and have no idea where to even begin.
If you're using gold for your currency, you might be interested in GoldIsMoney. Version 2.0 is in development and I'm looking for ideas of how to actually implement something to support Vault's bank command's. I'm considering something similar to your idea here, but I like to shy away from more commands. Anyways, check a look and see, it might be on the road to what you're looking for.