Can't see Del Potro winning a slam outside HC. He may not even win a slam again. This is a tough era. I could see Berdych winning a slam someday. Del Potro again as well. Tsonga? Nah.
Slams are not for everyone these days. Look how long it took Murray. We are not in 2002 anymore, an era where it was easier for the Johansson's of the world to win a slam.
This is not a tough era at all. Del Potro had wrist surgery and upon his comeback has had injury after injury and still there are only a handful of players who can stop him in big tournaments. Del Potro is an extremely talented player, but in a real tough era he'd not have had it so easy to get back to the top after what he's been (and keeps going) through.
The younger top (4) players are all great players, but they're not imposing all around talents like Federer. They excel in today's conditions, but as soon as those change even a bit (ie Madrid) they either boycott, whine or lose early. In no way, shape or form do they feel as unbeatable as Federer once did (and Nadal on clay). Fedal's decine/retirement will open up possibilities for a lot of players as Djokovic (and Murray) will never be able to 'guard' the Slams like Federer and Nadal did.