Another obscure fairy tale I'm considering adapting is The Golden Mermaid. There aren't many fairy tales other than Little Mermaid that contain mermaids, so it seems like an obvious choice.
Adalmina's Pearl, an obscure Finnish "King Thrushbeard" variant, also seems like an easy choice, as it involves a pearl and an anti-heroine with a very mermaidy-sounding name. The potential downside is Adalmina is a VERY intentionally unlikeable character initially, probably the nastiest princess in the whole European fairy tale canon. That could make her fun to write though, as Nixie in A Tail of Two Mermaids was a lot of fun to write too.
I also keep going back to Maid Maleen, because the title already sounds mermaidy, but that's a very dark story. I thought of Maleen's father cursing her to be a human for seven years instead of locking her in a tower.
Also researching both Cinderella and Beauty and the Beast/Cupid and Psyche variants from around the world. BATB/Cupid and Psyche are challenging to get right, as it requires a lot of attention paid to character development and nuanced relationship dynamics for it to not come off as Stockholm Syndrome-y and creepy. I tend to be wary of "East of the Sun, West of the Moon" variants for that reason.
As for Cinderella, I already did a pretty Cinderella-like heroine with Melinda in "The Servant Girl and The Sea Prince", and I feel Cinderella retellings are overdone, but the basic barebones of the story can be pulled in so many different directions. I like the Biblical Queen Esther story for having a Cinderella-like heroine who is part of a persecuted race who saves her people, but it could be cultural appropriation of some kind to make a direct link, although I can still do that basic theme because Cinderella can go in many different directions.