Djokovic to catch Federer atleast in slam count would have to do something like win 10 of the next 19 slams until the end of 2017 (so roughly half), then win another 2 or more after that, now aged 30 or older. While anything is possible given his current dominance and the lack of up and comers, it would be a real long shot at best. However if he wins the Grand Slam, and multiple slams at every venue, he could be seen as greater than Federer even reaching 14 or 15. The latter is still very tough, but actually not that unrealistic.
His chances of catching and surpassing Nadal are probably more realistic.