Nadal or Djokovic, fighting off the new guard.
I think either the 4 Slams will be split 50/50 between the usual suspects and either new Slam champions or current 1 Slam champions, or, that Nadal will have his final standout year giving another sudden boost to his Slam tally, before riding out the rest of his career mainly on 1 Slam win a year until inescapable serious decline.
I think some approximation of this is the answer. I really thought 2014 would be Djokovic's second multislam season, but after the Nishikori SF, I think he'll win 1-2 more over the next 2-3 seasons. Hard to imagine he'll have a better crack at Nadal at RG than he just had; hard to imagine he'll get the green SF opponent/graying finals opponent combo again at SW19; and hard to understand weeks later just how he lost that match to Nishikori at the Open.
I also have a weird hunch both Federer and Nadal have another Wimbledon title each in them (or at least another final in Nadal's case) - regarding Federer, I keep thinking about Pete falling short after deep runs at the Open from 1998-2001, followed by all the cards falling into place for him in 2002. I suppose that may've already happened for Roger in 2012, but I have a sneaking suspicion otherwise.
As for Nadal, he's due for one of those 2006/2011 easier first week draws at SW19, and should benefit from the extra week b/w RG and SW19 as much as anyone. I posted this the other day, but the list of Open Era players who've won multiple Wimbledons and made at least 5 finals is a pretty elite club:
Federer (7 titles, 9 finals)
Sampras (7 titles, 7 finals)
Borg (5 titles, 6 finals)
Becker (3 titles, 7 finals)
McEnroe (3 titles, 5 finals)
Connors (2 titles, 6 finals)
Nadal (2 titles, 5 finals)
That's the list, and we're approaching 50 years of Open Era tennis. So although Nadal's been a mess at Wimbledon overall since 2012, and was even wobbling against journeymen in the first week as early as 2010, it's hard to ignore that resume and say he's over and done with at the event. I foresee at least one more great run there for him as well.
Add to that two more RGs and another Open, and there's 18 for Nadal. Ironically, I think he never gets another AO title - thus ensuring (as it must) that the Federer-Nadal wars rage until the sun burns out.