JK Rowling once said in an interview that she thought about having Dudley have a magical child on platform 9 3/4s at the end of book seven, but decided that no magical blood could survive contact with Vernon Dursley's DNA. In a long list of ideas she got wrong about what should happen to her characters post-canon, I've always thought that this was pretty near the top.
Despite my reservations, this mostly takes the Harry Potter epilogue as written, but makes a few significant changes that the plot revolves around.
Apologies, I've been lax about moving the rest of this story here. Working on getting the rest up!