We are working on a couple of Fows for a customer who is using SharePoint for Vendor Collaboration.
I agree that this would be a valuable Flow. In the same line, I also think being able to create a new SharePoint site from a Template would be valuable.
You can create a custom site with the help of Plumsail SharePoint connector, which is a part ofPlumsail Actions(third party set of connectors for Microsoft Flow with a fair price).