In the unlikely event that they decide they want Q, but can't persuade John de Lancie to reprise the role (as his arguements aren't invalid), I could them approaching Keegan de Lancie as he'd be reasonably easy to tweak to more closely resemble his father than most, and even in the hard canon obsessives couldn't really say "but he's not Q!" because he is Q (if not by "Q Junior" in the RW for ease of reference).