I challenge anyone and everyone to write a Harry Potter crossover with Percy Jackson & the Olympians WITHOUT any of these things:

Demigods being sent to protect Harry

Any new teachers

Demigods enrolling in Hogwarts

People joining the Order

Harry being the child of some god or another

Any of Harry's friends being the child of some god or another

Reincarnation

Percy and Harry (or any other two characters) being related somehow

Anyone being younger or older than they would already be

Adult HP characters or their children

Dumbledore knowing everything because Dumbledore (it gets old)

Everyone ignoring the fact that Ron has a prissy brother named Percy (I mean come on! No? Okay...)

People of authority keeping secrets about the other magic world from the main characters

Pairing anyone with anyone whose relationship with them isn't canon (personal preference)

Constantly going on about how the main heroes are exactly the same

OR

Being just a few short paragraphs long. 'Cause that's boring as Hades.