I think it would be great for every purchase of mine to round up to the nearest $5 or $10 increment and transfer the difference into savings, or have it be percentage based, etc.

Having payments from clients automatically set-aside tax money would be great, too.

This sounds like a great idea!

Personally, I would rather rely on self control and not have my card forcefully decline transactions. But maybe that's just me!

That's the point though. Everyone has their own somewhat unique requirements.

A traditional bank can't cater to every persons individual needs/wants.

It would be nice to have somewhere these scripts are discoverable so you can use/tweak other users ideas.

Ability the decline transactions is my dream. Personally I want to approve every charge as it happens rather than have to scan my statements for anything unusual

While I agree to some extent, if we ever want to get rid of welfare etc and move to a basic income for everyone one of the problems could be people mismanaging their basic income leading to people still starving or having other financial troubles.

Something like this API may make basic income more feasible if we could ensure that people aren't able to blow their budget and go hungry even with basic income.