Don't think Djokovic will surpass Fed on clay. Djokovic has on RG final. Fed has 5 and a title. Several other big clay titles and in the clay GOAT era. Djokovic would need to win 2 RG titles to have a chance. If he does it when Nadal is gone it won't mean that much. He shouldn't have lost to Roger in 2011 and to Nadal this year at RG. That pretty much says he will never surpass Roger on clay.
Djokovic might win one RG but he won't reach five finals there. When you almost lose against the likes of Tsonga on clay
you know even winning one RG title will be difficult. He will probably win a very tough RG title like Agassi and Federer. The only player who had it easy when trying to complete the Career Slam in the Open era was Nadal who only dropped a set at the USO in 2010. But that is the exception. RG is usually the most elusive slam for the 6+ slam winners.
Becker, Edberg, McEnroe, Connors and Sampras failed to win RG.
Lendl and Wilander couldn't win Wimbledon.
Borg never won the USO.
McEnroe and Borg skipped the AO for most of their careers and of course never won it.