How about having central repository like:

repo.scheme.org or packages.scheme.org linked to git repo that will only host
manifest files?

It would require extra step to first download meta data file first and then
the actual files but it would make the name:

(import (repo.scheme.org/user/repo repo))

not changed

it can be structured like NPM and GitHub, you will be able to create Pull
Request to create user (just a directory with first repo, or with .gitkeep -
git doesn't like empty directories because it track only files).

This will have few benefits:

1. Central repository with only compatible packages
2. Code can be hosted everywhere, the manifest file can even have FTP and HTTP
protocols in addition to git
3. You can move repo from GitHub to GitLab and only need to change the
manifest file.
4. If there will be a lot of packages We can have search for them

There will only be problem with versions but they can be outside of Manifest
file that will be only a pointer to repo. Manifest can be simple like a
symlink that point to compatible package.

Things that probably will not change:

* Name
* Description
* Author or authors
* License
* Tags
* Pointer to source code