I don't know if there is an "offical" place for it. I know that it sometimes takes some time for http://getkirby-plugins.com to get updated. It also does not have that many plugins in comparation to this forum.
The best place should be one that it's easy to add new plugins, easy for an admin to approve them and as little locked in as possible. A wiki is the closest thing I have in mind and this is a wiki post.
Some sites uses github where anyone can add pull request which then get added.
Many admins is probably one way speed things up.