This sounds like it could be fun, in a ridiculous kind of way.

Yeah, 40k and stuff would utterly wreck Percy Jackson's earth and probably everything Earthbound has [i]in a semi-realistic fight[/i], but I reckon the only way to pull off a crossover as insane as this would be to throw the rules out the window and just go with whatever seems awesome. Instead of trying to ruthlessly beat each other, we try to have a cool battle with as many explosions as possible! 

Anyways, if this thing ends up getting started somehow, then what the heck, I'll stand for the Homestuck fandom.  That pretty much ensures ridiculousness!