Save as draft functionality works really well with authenticated forms but there isn't an in-built function for unauthenticated forms. This means the workflow designer has to code a workaround.
This idea is to enable the use of the save as draft feature where the user would be emailed a link to their incomplete form.
My suggestion of a nice user experience would be if the anonymous user selected save as draft, there are a few fields for the workflow designer to use to construct the email that will be sent to the user similar to a cut down task email. E.g. sender name/to email/name/body (with an insert hyperlink feature or switching on the standard footer).
The system would need to check if they selected save as draft that the email control was mandatory/populated.