Federer: probably has another Wimbledon in him, maybe even another USO if the cards fall right this year
Nadal: no idea how he'll come back but it's hard to see him not winning another RG or two and maybe a Wimbledon
Djokovic: depends on health but he could end up winning the AO, RG and USO multiple times if Federer and Nadal retire/decline enough over the next couple of years
Murray: will probably win Wimbledon at least once and will probably pick up another HC slam or two
Del Potro: hopefully will pick up a French title and another USO at least
Del Potro: 2-4
People going on about how Fed/Nadal/Djokovic are only going to win 1-2 more each - who else do you see winning the majors???
Surely someone's going to breakthrough soon enough?