Del Potro will not win another Slam in his career. That's obvious.
I also think he will decline pretty early. About Masters 1000, the chances go like this IMO:
Won't win any Masters before retirement : 60%.
Will win 1 MS tourney before retirement: 39%.
Will win 2 or more MS tourneys before retirement: 1%.
There are right now 5 players who are clearly better than him. Year-end #6 for Delpo is kinda optimistic, but possible. Berdych should end this year as #6 IMO, unless he fucks up like the last month and a half. My prediction is that JMDP and JWT will fight for #7. Probably (and hopefully!) Del Potro will edge it.
Slams in 2013: 3 QFs and one 4th round exit.
I'll give you three players that will be ranked higher in twelve months than they are now:
Djokovic, Murray and Del Potro.