Clearly Novak is the best player right now and will continue to be unless at least a couple of the following happen: Rafa re-finds his peak form, Novak takes a step back and/or becomes complacent, some new young gun rises to superstardom. But he really seems almost unbeatable right now.
Given all that, it is hard imagining Novak not being the premier player at least through 2016. After that the field will be more open, with young players at higher levels and Novak starting to slip. So 2015-16 is his time to rack up some serious accomplishments.
The biggest hurdle for the calendar Slam is obviously Roland Garros. If Novak can take that, then it is a distinct possibility as he remains the favorite at Wimbledon, unless Andy Murray discovers a second serve and/or Roger Federer can find his highest level. He is also the favorite at the US Open, but I wouldn't be totally surprised if someone like Kei Nishikori, Grigor Dimitrov, or even Milos Raonic puts it all together there. It would be high drama to see Novak enter the US Open with 3 Slams under his belt, then lose to someone like Grigor. It might signal a regime change, like Roger losing to Rafa at Wimbledon in 2008. But that might be 2016.