Here's what's got me thinking:

First we hear that Aunt Petunia will prove to be very important and she's going to do something that's a big deal in the last book. So that makes me think she's a squib or a witch in hiding and that she's going to perform some magic.

Then Jo comes right out and says somebody is going to perform magic very late in life. At first I thought it would be Petunia... but it can't be her because Jo said that Aunt Petunia is not a squib AND she's not going to perform any magic late in her life, therefore she isn't a witch either. So now i'm thinking the person performing magic late in life is going to be either Flint or Dudley.

Dudley is Harry's first cousin. You never know... he just might be a wizard!