Try to blog once a week, describing 1 package that is clearly missing on Hex.
You create a complementary project on GitHub called something like "missing-hex-packages". There you create 1 issue per blog post.
You can ask people to participate in a discussion on the issue and eventually link their implementation of the described package in the issue's comments.
Use the mailing list and/or sites like elixirstatus.com to raise awareness for your blog post series and increase participation.