We've added a new configuration option of 'Ignore required field validation on save' in Form Settings.  

When enabled, users that have access to the Document List of the app (as an Administrator or via Conditional Permissions) can save the documents without completing the required fields when they edit a document from the Document List. This enables shared responsibility for drafting documents by allowing multiple contributors at different times.  This behavior mirrors a creator saving a document prior to submissions, or a workflow reviewer saving a document before completing their task or approval.  The user can update the document but it will still prevent the Approval, Acknowledge, or Complete Task action until all required fields are completed.

Please note, this will allow someone with access to edit a document to remove data from a required field on an In Progress or Complete document. This may be appropriate in some cases, but this configuration does put more responsibility on document editors to ensure they are not clearing critical data from a document.