Neville is the wizard to gain power later in life

[katssirius]

No: HPFGUIDX 150816

I always thought Petunia was the closet witch or at least a Squib 
but JKR refutes that on her site.  My next guess was Dudley and 
although I can make a good argument for it I see one big problem.  
JKR considers magical ability a gift she gives to the characters she 
likes.  The better she likes them, the more they have.  JKR does not 
like the Dursleys and I hope she does some consciousness raising 
about making Diddykins and Vern fat, but thats a tangent.  

Anyway, I think she means Neville.  Neville is on the way.  I was disturbed by his absence in HBP but I think Neville has a date with destiny and he needs the power to win.  I see him as the next Dumbledore 100 years from now.  He has the wand and for the first time in his life he has a proud, supportive family.  These are life changing.  Neville is a hero waiting for his hero power.  I think he is the exception that JKR mentions of magical ability coming later life.  Remember his family worried he might be a squib.  He has made progress in courage with the help of his plant now the skills and power to use 
his courage.

By the by, of course, the villains have magical power too but that is 
a necessary plot device as opposed to a gift from the author.

katssirius