This is pretty straightfoward- this would sort out a lot of existing problems-
1. It gives the ability to restore just a part of a site ( restore to a new project, then use cross-site copy & paste to pick the elements you want ), including assets like site-wide code, etc.
2. It minimizes the unnecessary reset of the CollectionID's and ItemIDs, and consequently the breakdown of all Zaps & integrations.
3. It gives the ability to compare two different versions of the site side-by-side in client meetings.
However, if the site is based on a purchased template, then this other wishlist item needs to be considered as well;
Allow the ability to clone paid-template projects, without paying again for the template (until hosted).