probably something like
Federer 17 >>>> 18/19
Nadal 11 >>>> 14/15 (really I mean at the very least he's got a few more Roland Garros's in him probably, but could possibly challenge at all slams if comes back as strong as ever)
Djokovic 6 >>>> 10 once again, although obviously Novak is the best player as far as form and consistancy these days, his style does seem to be grinding him down. just as an example, he looked to get more tired at this Australia Open frequently than I have seen in the past. and people lose sight of how hard is it to win ONE slam, let alone multiple more. too many unknown variables. three or four more is realist, but those guessing something like seven more are being very short sighted
Murray 1 >>>> 4 How can you not see Murray winning a few more? he's one of the fittest players on tour, has made the last 3 grand slam finals, etc. even if Djokovic were to become his bogey man (which I doubt, I expect them to continue to share wins when they face each other), if Novak gets knocked out early in some grand slams tournaments it would make Andy's chances pretty good.
Del Potro 1 >>>> 2 hopefully this becomes more optimistic (if Juan can gets his fitness improved and to a degree his confidence). I definitely think at the very least he'll get hot one tournament at some point and win some. however for all we know, when he's like 27 he'll have a prime and beat everyone everywhere. given that Del Potro has had numerous good deal of times off for injury, and the fact that he isn't the guy who is out there running insanely killing his body, it's certainly possible that he could have a late 'prime'. a weakening tour wouldn't hurt either... I mean he's only, what, 24 years old after all
Tomic --- he will win some