In an idea world, something like a bank. The problem is that banks have services and structures designed to enrich the bankers, not be useful to their customers. This sort of stuff isn't worth their time, and the digital banking infrastructure is too much of a mess for apps built on top of it to really shine.
I have a really hard time matching your stated inevitability with the fact that my bank has had that exact kind of service since I have bank accounts (all of them, except Citibank that I tried once... so all of the local ones).
But, of course, they don't interact directly with the US banks.