It's possible that Djokovic will surpass Nadal and Federer as far as Grand Slam wins. Djokovic is still strong and he was able to overcome the psychological barrier last year and started to win again. The only hindrance is that he gets injured again, but I guess he learn his lessons last year and knows how to take care of his body more so that he can continue to win more trophies.
I'll second that it's quite possible Nadal just returned a few days back, and he's already had to withdraw from the tournament his injuries seem to be persistent. Also while Federer had a good match recently I don't feel he'll be able to stop Djokovic. I feel that if Djokovic can maintain his fitness, and perform consistently he can be at the top there's no stopping him but I feel he'll face some competition from youngsters who'll try to beat him.