One I've held onto too long and have recently told myself to let it go was a crossover of Waiting for Godot and Pokémon. However my brain just could not plan it out beyond the initial meta joke of "fandom in denial about TPC respecting their time/money".
As for ideas I'm still clinging to...
Crossover of YGO: Duel Monsters and CRAWL. I have a bunch of ideas doodled out and notes but don't have it properly organised. The narrator of CRAWL is absolutely King Thief they are one and the same to me lol.
Writing some wlw for Sakuna of Rice and Ruin. Sakuna and her best friend Kokorowa have a lot of fun potential! (I also need to state that I'm the only person to have written anything for this game in English. Pain.)
Pokémon GS/HGSS but specifically about the power the player (once upon a time) has over the rival's name and using that as a tool. Pokémon but what is the golden/pure heart Ho-Oh is looking for (which is now wrapped up with the other idea explaining what had happened to the Brass Tower). Fixits of all of the massive plotholes SwordShield and ScarletViolet have.
Fixit of the massive horrible mess of Pacific Rim: Uprising. So many good ideas that should of been separate films but instead they choke on each other in one.
What if Jurassic World kept the heart of the anti-capitalsim of the original book and first film. What if we explored the coolest idea in Jurassic World that was just a news clipping shoved in the background. "the worth of allowing once extinct animals go extinct again" and compared to other animals we once thought to be extinct and we are studying in real life today.
(Insert 50+ more ideas here lol)