Something I was planning on building (and sort of have for the hackathon, is the ability to put transactions into catégories (did that thanks to a smart contract storing the txid and the Category which showed up on the user dashboard. But the main idea was to have the ability to submit a message (converted into hex string as tx data) when sending one to a user, where he could supply a category and message within the transaction. Which would be displayed either in explorer or on a separate page where users could see sent transactions and or classify them. A data structure would need to be thought of and it would have to become a norme.
Really would expand the possibilities and use cases for banking like applications.
Plus it would resemble what banks already do, the downside being tx data is public. So anyone with the will power could see why you sent that transaction…
Just an idea.