Actually, there's no one coming through, is there? The Joker and Andy could mop up over the next four years.
DelPo will improve, although I don't think he'll ever reach his 2009 standard. Too one-dimensional anyway, the top guys have his number.
Tomic, Raonic, Harrison (both of them) and Nishikori will be threats but I don't see any of them ever getting near the standard of the current top guys. I can see Raonic being a little like Roddick, getting to the odd Slam final/title when he's on form.
None of the juniors really shine. Quinzi maybe. Saville and Peliwo will probably become top 20 players (Peliwo reached all four Slam finals this year, and has dominated the juniors to a similar style Monfils did, so I can see him following a similar path) but there's no stand-out players who seem a step above the rest.