I like using sticky fields, e.g. for the source while making cards for a specific article, book or paper. But then e.g. I stumble upon a new English word, so I create a card for that with a different note type and the sticky source, tag etc. are gone. Often I go to the Anki browser to copy-the field contents.
One solution would be to allow for multiple add-sessions, each with their own sticky tags and fields. Another solution for my particular problem would be having some kind of tag and field history similar to the browser query, e.g. accessible via a dropdown. But implementation wise I expect multiple Add-windows to be simpler, though I’m not sure. We could have shift-clicks open a new Add-session for example and by default when the user clicks the normal “Add” (without shift) focus the most recently focused one.