It would be great to be able to edit a document without loosing everyones signature that has already signed or the signature fields that you have put in.

ability to undo editing a completed document or recover the completed document

PandaDoc needs to have the functionality to update or "amend" fully executed documents. There are items from original signed document (dates) that can not be changed. However, exhibits/amendments/addendums must be added over time. Current state of restarting document from scratch is not an a viable solution.

When all signers have the document available at the same time, the signer should have the option to go back and edit their work. Because other's signatures were not dependent on one another, that should be an available option.

Option to retrieve sent document once editted

Option to retain signature of the signed document during editting.

When a document is sent there is not a way for the document creator to edit the document without marking the document in edit mode. So for instance if someone signs and suggests that a file is added to the document you cannot do that without it being in edit mode. Another example is if some people have already signed off on the document and it was brought to the document creators attention that there is a type in the document or a name mis spelled the document cannot be edited on the fly to correct these items without putting the document into edit mode the signing process starts all over again.

Probably not legally binding... customers would like to be able to change the content of a document after the document has already been signed

This seems to defeat the purpose of a contract. I don't think this feature should exist, or it wouldn't be a clean audit trail of what was agreed to. If you edit after signing the signer should have to re-sign your edits.

Possibility to edit fields once the document is sent and finalized from Sender's side.

The possibility to resend the document saving the already existing signatures.