Another Crack Crossover Idea: Percy Jackson as one of Ryland Grace's students. (Or any demigod really) + crackship Ryland Grace x Apollo (don't take this too seriously pls 🙏)
Demigods always struggle in mortal school systems. More often than not, they don't consider the children's ADHD, and dyslexia and the demigods aren't getting proper understanding nor accomodations from most of their teachers which obviously results in most of them dropping out, keep moving, and generally not learning at all. And then Grace, probably undiagnosed audhd, takes his time to accomodate them, because be knows how they struggle in a system that's not built for them. There are other teachers and higher ups in tbe school that think these problem children are lost cause, too hyper, too lazy, etc.
He easily becomes their favorite teacher, making science and studies genuinely interesting instead of a chore, or something that's personally out to get them.
And then the whole Astrophage thing happens. The sun is dimming. And somehow the Greek Pantheon, or the divine realm in general are connected to it and are also having trouble with it for reasons that I am not that creative to come up with.
The plot of Project Hail Mary still happens, Grace gets forced in the ship, meets Rocky, etc.
(Mid way through typing I thought of another idea and thats where the Grace x Apollo ship comes in. Sorry my mind is vv scrambled rn)
Whilst the gods are panicking, the launch is announced in TV, and Apollo, disguised as a mortal, sees that scientist Ryland Grace was one of the astronauts and his first reaction was "This mortal is kinda cute", then "Wait.. Isn't this my daughter's science teacher?" He has been doubling his efforts in being more involved in his children's lives(no matter what Zeus says) and he remembered her mentioning a favorite teacher of hers. Ryland Grace.
Then, the beatles come back and the the sun is slowly starting to get brighter. Everyone is in disbelief that a mortal (who is not aware of the godly world at this point) can solve a problem that the GODS have been trying to solve. Apollo at first is as shocked as the others that a MERE MORTAL MAN. Then after his initial shock, he starts to feel something else.... Artemis is getting tired at this point considering her brother's dazzling track record of tragic lovers.
Imagine Apollo rolling up to Erid in his chariot being like
Waitt in the Riordanverse, multiple pantheons coexist. Like the Greeks, the Egyptians, the Norse. Imagine if Eridians have/had various religions and also have a few pantheons running around and now Grace saved two suns, two planets, and two(and more) pantheons probably.
(Ryland recognizing some of his children students at Camp Half Blood is a bonus)
Lets just hope that Grace doesn't turn into a plant.