I run my own instance and I have created some communities that I loved back on Reddit. I searched and no similar communities seem to exist in the fedeverse yet. Obviously there is nobody visiting them right now, so how would you go about trying to promote it to attract some discussion?
Or post them to reddit. Some subs are actively asking themseld if and where they move. Of course only the willing part, but a simple: "I created this community, see yea" could be enough
Honestly just create them, and don't leave them for dead (give them content). That way they show up in searches on peoples instances and places like https://browse.feddit.de/
Be sure you have searched in the feddit.de or lemmy.ml searchbars rather than your own server's, since those will only populate with communities you've checked out.
Other than the wow this lemmy exists mentioned above, find communities in the general topic, example for F1 racing find car and hobby related communities. Or post a meme with a related topic and promote your community in a comment.