JKR would probably be mobbed by HP fans if she killed him off in the last book.
At best she'll make him disappear, borrowing from King Arthur's myth - to send him off into the mists, to return when next evil should rise again.
(Or worse yet, she'll send him back in time to grow up to become Dumbledore.)
*pingus maxima*
You'll face angry mobs with pitchforks if this happens. I for one will blame you for giving her the idea. Watch your back.
"`All right,' said Harry; and this time he vanished quite slowly, beginning with the tips of his toes, and ending with the scar."
Having him disappear, will the implication that he's roaming the Earth invisible or something like that, would boost the legend among child fans, who'd promptly start playing games where Harry turns up in their own neighborhood.