Just to point out... Rowling said that someone who "can't do magic" will use it. So it really doesn't HAVE to be a squib (although I would like it if it were... I think the whole idea of squibs is sad

.) But it could definitely be Hagrid. However, that's not nearly as exciting as the idea of it being Petunia! I personally don't think she's going to just spout magical abilities, especially after all this time, and she has had some stressful moments in her life (think of all the times she screams in the books

). Maybe she will use a portkey, or floo powder, or the pensieve, or something that doesn't require a wand. Maybe she will turn out to save everyone and force the Dursleys to understand Harry a little better!!