I don't know if people have really looked at it, but I started writing a fanfiction that was a crossover between PJO, HP, and TSC, but I haven't really updated it in... forever. I'm going to take it down because it isn't really fair to have it up when I know I will literally never update it again, so I'm going to put what I had for a plot here, and if anyone's interested, you can write it.

Background (logic stuff, any tweaks that were made, etc.):
You know how in Greek mythology you can choose to have your memory erased and have another life? And how if you get into Elysium three times you then go to the Isle of the Blessed? Okay, good, you need to know about that. If you don't, go research it. Or just read PJO again.
Next thing you need to know: Ever heard of the Multiverse Theory? No? Basically it's the idea that there are infinite universes, one for every single thing possible. Some are really similar and can be called parallel universes, like the TDA universe and Thule. You might want to research that a bit if you didn't already know about it.
So basically, in the fic, when you die, no matter what universe you live in, all the souls go to the same place. The "Underworld" (it can be called many different things depending on the religion, universe, etc. that you live in and believe in, and also look different) is connected to all of the universes, all the infinite realities. When you are born into a new life, if that's what you choose, you could go to any universe. It's chance, really. For example, you could be born into a universe where there is life on Mars, live as some alien species, and then when you die, go to a universe where dogs are in control of everything like humans are here, and then when you die again, you could go to another universe. Anything.
There's also the idea that when a book or movie or something like that is created, it's really a reality that the creator dreamed of, and their mind is somehow connected to it. There would also be a universe for every fanfic out there, and so on. Dreams are the closest state that you can get to to being dead, which is why when you're asleep, you get glimpses of other universes.
Now that you know all that stuff, here's what the logic in this fanfic is: When Will Herondale does, he went to Elysium. It had been the first time he went to Elysium. He chose to be reborn; this time he was born as Harry Potter (the HP timeline would have to be tweaked—everything scooted back so Harry was born on July 31, 1950). When he died, at age 41 (1991), he got Elysium again. The third time he was born, it was as Percy Jackson. That is when this will take place. 2013, when Percy is 20, Albus Potter is 37 and has children, as well as some other characters from his generation; it has been one year since TDA happened.
Oh, yeah. More important info: if you were to imagine the multiverse, it would basically be a giant and infinite thing if jello, with universes suspended in it. Or, as Jem explained in CHoG, a honeycomb, where each section is a universe. It'd basically be like that, except that technically all of the universes are next to all of the other ones, which cannot be logically explained using science and math, but who really needs logic, right?
So there's a sort of "magic" barrier between the universes.

Beginning plot:
Percy gets attacked by a monster, nothing unusual, but the wound it inflicts on him isn't like a normal one (also, when it "dies", it doesn't explode like normal; it's sort of sucked in on itself and then disappears. Like when a ship is travelling in Star... Trek? Might be Star Wars. I don't know). The monster was actually a magical creature from the HP world, and the mist altered it to look like something Percy was familiar with. He starts blacking out randomly, getting visions of the HO world from Harry's POV. Annabeth and he go to Chiron, who in turn goes with them to the gods. Because they're kind of the "rulers" of that universe, they know a bit about the multiverse business; Hades and Hecate know the most, Bri BF the god of the Underworld and the goddess of Magic. The cause of the HP creature was a rip in the "jello" between HP and PJO—the creature wandered through, attracted by the faint aura of its homeworld that Percy gave off (because he used to be from that world). The gods reveal a bit of info to the demigods but not much. On their own, they discuss that in order for the "rip" to be closed, everything from the HO world would have to be in its own world. Unfortunately, because Percy was having the visions of the other world, he counted as part of the HP world and the PJO world. The only way to restore balance would be to kill Percy. Then, because everything is already unstable, another rip happens! Yippee! This one is between the TSC world and the PJO world. Again, demon is attracted to Percy. Percy gets wounded. Percy starts seeing his life as Will as well as his life as Harry. Demon from TSC finds its way through PJOU (PJO universe) to HPU. A rip between HPU and TSCU is opened. Now there are three holes—one between HPU and PJOU, one between PJOU and TSCU, and one between TSCU and HPU. Because of how long the tips are there, they get wider and wider. The only way to shrink them is if everything is in its respective world. That didn't happen fast enough, so eventually all three world become one. For example, there is one New York, but both the New York Institute and the entrance to Olympus are in it. Or there is the UK, and both Platform 9 3/4 and Kit, Jem, and Tessa's house are there. Get it? Hopefully. Anyways, Shadowhunters, demigods, and wizards start running into each other, and, naturally, chaos ensues.

In Europe:
Kit, Jem, and Tessa somehow run into the HP peeps and then, after getting to know them, decide to investigate and figure out what the heck is going on. This would lead them to having to go to the Scholomance because of its access to the Spiral Labyrinth and stuff. There, guess who was already assigned to investigate it? TY!!!! They would obviously need to work together 😏. Maybe the Devon Crew (yep, I'm calling them that) could go to the train station to take a train to Romania and then meet the HP peeps there. Anyways, that happens, and Kitty eventually happens because it just has to.

In America:
For some reason the Blackthorns and Emma and everybody else in LA have to go to New York, or not. I dunno. If they do, then great. Somebody goes on patrol and gets overwhelmed by demons, then a few demigods (maybe some of the seven?) come along, think they're helpless mortals, and help them. Maybe one of the Shadowhunters or demigods gets injured, and whichever it is, the others take them to their place. So if a Shadowhunter gets injured, they go to Camp Halfblood, but if a demigod gets injured, they go to the institute. And I don't really have much planned out after this. Or I forgot.

Oh yeah! The child of Athena that I made up. I have no idea what I was doing with her. I think I was going to make her sort of a demigod, but not really a daughter of Athena. Like, if one of her parents was a god+. One of the beings that oversee the Multiverse. Anyways, they were all going to eventually get together and have a big vote on what to do, but even the gods, the Clave, and the Ministry of Magic could only come up with the solution of killing Percy. So that is what they make the decision to do. Until the new character interrupts the voting thing when they decide it and tells them how to solve the problem without killing Percy. And yeah.

If anybody wants to continue it, just let me know. You can change things completely or use these ideas or whatever you want, I don't care. Go nuts. But for now, I'm unpublishing it.
~K3