Append a "list" Custom Field
Append/Add an item to a "list" custom field.
This would be SUPER beneficial for displaying content with liquid in emails so that you don't have to bloat your app to set email content criteria.
Example: I have 20 different downloads listed on a custom field list. You have previously downloaded #1 and #3.
When you download #5, the tool would update the custom field to show 1,3,5.
This field would then be used with liquid to display email content dynamically.
While my example is based on "downloads", the appended list could be used for multiple types of criteria at once (as they hold 499 items).