This is not only necessary for the new packaging method for elementary OS, but will have some incredibly powerful advantages beside that:
Users can literally compile and test the app with a single dependency (io.elementary.Sdk)
You can create git release artefacts in the form of the .flatpak bundle
Users can install and uninstall the app with virtually no repercussions
This does require some research into how disk image reading/writing can be done from the sandbox, but I have no doubt that it's possible. It's also better to handle this early so all the researching/porting isn't left until after the app is release-worthy.
This is not only necessary for the new packaging method for elementary OS, but will have some incredibly powerful advantages beside that: