FR: Make custom metadata "requireable"

Would it be possible to make certain custom metadata required so that new documents cannot be approved/metadata-curated when these fields aren't filled?

It probably makes most sense to do this only based on the presence of a tag.

Use-case:

Or: invoices in general need a person to approve the invoice. I.e. when an invoice comes in (via IMAP or scanner, it gets tagged `invoice`. To be able to clear the "new" status, a person needs to be assigned for approval.

This will likely come in handy with workflows, too.