Yeah, that's a cool idea. Only instead of a ghost at hogwarts, Fred could be the ghost of the joke shop. And so George and Fred could still pull off their jokes.

Also, I wondered if Hermione ever went back to Australia to get her parents. And, what ever happened to the Dursley's? If Harry can forgive Snape and name his child after him, I could also fathom him having some forgiveness towards the Dursley's. After all, JK did redeem Dudley somewhat early in DH. Something else to think about.