Oooh, this is relevant to my interests! Some banks do have APIs -- U.S. Bank, Wells Fargo, Bank of America, and Chase are a couple that spring to mind off the top of my head -- but in general they are not really aimed at developers building for specific consumer use cases, which kind of makes sense, since most people don't want to build, say, a budgeting app that only works with their own bank. A more typical customer of these APIs is fintech platform companies like Plaid that will work as a middle layer and make this data accessible via a more standardized API. (Full disclosure: I work at Plaid. We also have our own open API standard, Plaid Exchange, but obviously not every bank is using it.)

Anyway, to answer the question, since it would be something just for my own bank it would be a service that I would use for myself, rather than something that would be a business. So, this is small, but a few months ago I was trying to find a good way to split the rent with my boyfriend, and even though it seems completely bonkers, in the year of our lord 2021 I still cannot find a service that will allow me to make free, recurring, peer-to-peer electronic payments. (My bank offers free peer-to-peer electronic payments with Zelle but doesn't provide recurring functionality; and they offer free recurring bill payments to individuals but only via check, not electronically.) So I might build that.