I use the Gallery field a lot as I prefer it as a loop model for image selection over the Repeater field, but I often want to attach custom data to those images; a crop position, a page link and so on. I’ve written code to store these with AJAX with my own custom function, and I amend the ACF core files so the Gallery field returns the data I want it to. It’d be nice to have this built in (but then again there’s loads of things I amend in the core every time I upgrade my ACF >.<).
It could possibly work by whenever a user creates a field group that is conditional to media attachments only, the fields in that field group (if text or textarea only maybe) appear not only in the Gallery fields list view on the edit screens, but also in the image object that is returned for each image in the gallery field array.
Another much needed upgrade of the list view of Gallery view is the editability of those bits of info… they’re there but you can’t click them. When i’ve demo’d the Gallery field to clients before it is natural for them to click into those bits of info to try to edit them, but the edit popup actually opens a modal.