Ask HN: What to build using a bank account API?

Given the public beta of Teller has been announced today[1], what useful apps could be built using their API to pull data from your bank account?

I've previously built an open source web app to visualise transactions from a Ledger CLI .dat file[2]. This could be automated using an API to create a dashboard of your financial transactions. What other useful tools could be created from this data?

[1] https://blog.teller.io/2017/06/12/the-api-for-you-bank-account-is-here.html

[2] https://github.com/slashdotdash/node-ledger-web