I don't see any of the current generation of youngsters challenging the big 3, in a way they're unlucky - Nadal, Federer and Djokovic are just that good. Even Djokovic, taking into account historical competition. I haven't seen much of Zverev but his win over Novak in the ATP finals was a good one. The problem for the up and comers is that the big three know how to peak at the Slams, that in itself is a skill that needs to be mastered. Djokovic is going to be very difficult for anyone to stop next year. Nadal's at risk of potentially being overtaken by Novak in the next 1-2 years (in terms of Slam count) if he can't pull out victories at non-French Open Slams.