Our good friend Rumple_deWriter used to post these lists from Authors Publish. Rather than pasting the list in a post as he used to, which broke the links, I'll just link to the list on their site. There's quite an assortment of themes in here. I see some I might consider trying. Some include:
Letters I'll Never Send
Aquatic Horror
Future Cats (so s-f about kitties)
Lovecraftian Folk Horror
Queer Weird West
An anthology about Baba Yaga
and more
» 39 Themed Calls for Submissions for December 2021 (authorspublish.com)