It would be great if we can reuse the same source used in Ubuntu source packages as part sources so we can incorporate Ubuntu & Debian patches and avoid using the source from upstream.
Well you can do it either way really. For the deb package you can download the tar.gz that was used to make the snap and then bundle that easily enough. I would guess it makes more sense though to pull from the source directly given they will be the most up to date and it wouldn’t really make a whole lot of sense to build a snap if it’s already available in the exact same form as a deb.
With the effort to make Ubuntu sources available via git, I think snapcraft.yaml could simply be pointed to the Launchpad git repositories. For the fully patched version, you could use the “applied” Ubuntu git repository branches.
Right now this is only available for main and is experimental, but the plan is to ramp it up for all packages eventually.
Is there any reason this won’t work? If not, then this wouldn’t need any new features in snapcraft, I don’t think.