The hope is that the repo would be open so anyone could clone it if the original project was being unethical. Similarly, users are free to modify the lists when posting, or maintaining their own forks so they can benefit from updates but keep their own structure.
We would need some rules about how the list should be ordered, and what content wouldn’t be allowed. For apps/extensions that’s simpler, you can order by installs and have a threshold (ex. Top 5 are listed prominently, rest are listed in a section below, ordered alphabetically). For instances/communities this is a little harder, and I don’t know what the best way to handle it would be.
For the second point, right now the alternatives are also single repos, so it wouldn’t change much past the structure.