So Djokovic is 5 years older than Nadal, 6 than Federer. Seeing as Federer truly came onto the scene in a big way was 2003 at Wimbledon, that's bumped back to 2009 Wimbledon. Before that he has next to no impact.
Djokovic's 2006 would coincide with 2000. Djokovic in 2006-2008 RG had a streak of losses only to Nadal, so it stands to reason he'd do at least as well in 2000-2002. I'd say he picks up one of 00/01 RGs and also 02 RG. 03-04 he had those losses, so I won't count any RG as his. However, 05 is interesting. He meets Nadal at this RG in his 2011 form. I would say that Novak wins this historic encounter. I would also say that this probably changes their rivalry, as Nadal couldn't snowball against Djokovic mentally like he did against Federer. However, I digress. In terms of RGs, I would say Novak wins 00/01, 02, 05, loses 08 (nobody beats Nadal there), wins 09, and never wins again. 5 RGs.
Djokovic also picks up AOs. 2002 easily, then perhaps 05 against Safin? Tough to say in 05 Safin vs 11 Djokovic. Would be a classic though. Takes 06, 07, 08, perhaps 09 but I'll give it to Nadal, and then he fades. 2004 Federer beats 2016 Djokovic, then there's no way he beats 2005-2009 Fed there anymore. 5 AOs.
At Wimbledon, it's tougher. He fails to win Wimbledon once Federer arrives, in 09, so he has from 00-09 to take as much as he can. In 08, does he beat Nadal? Unlikely, as that's 2014 for Djokovic against the best grass Nadal. 07, does he beat Nadal? Possibly. Depends a lot on draw, but I'd favor Nadal in the match. 06, this one Novak wins. Nadal was the biggest hurdle and Novak would play well enough to stop him. 05, Novak plays in 2011 form. No one beats him. 04, Novak plays in 2010 form, so Roddick beats him, probably, or Hewitt - still, he's got a chance. 03 is a great chance and the competition wasn't great but neither was he - 50-50. 02, it would be tough to beat Hewitt, especially since 08 Novak lost to 08 Safin who lost to 08 Federer, but possible. 01 is another possibility, but is he beating Goran in his 07 form? Probably not. If we add all that together, it's between 2-7, but around 4 if we account for upsets.
USO. 09, he's facing Delpo in his '15 form. Possible, but tough to say. 08, he's facing Murray in his 2014 form. 50-50 to say he loses like he did to Nishikori in 2014. 07, he faces Roddick or Davydenko in his '13 form. He wins. 06, faces Roddick in his 2012 form, but no exhausting match right before. He wins. 05, he's in his 2011 form against Agassi. He wins. 04, he's in 2010 form against Hewitt. He wins. Loses 02-03. Hewitt probably wins 01. 3-6 USOs. My guess is 5.
Djokovic would have around 19 slams.
Nadal would lose 05 RG, but gain 07 Wimby until 2009.
Then 2010 happens. Federer would have won Wimby 09, then absolutely pushed out Nadal in 2010. Federer takes Wimbledon and USO, fails at RG.
09: Wimby
2010: AO, Wimby (probably, but maybe Nadal?) , USO
11: AO, Wimby, USO
12: AO, Wimby, USO
13: AO, Wimby, USO (At this point I'll take one of these last 9 slams away because he'll get upset somewhere by someone (perhaps Stan at AO 13, but 70-30).)
14: Perhaps loses AO to Stan, perhaps not. 70-30. Wimbledon he wins again, USO he wins easily.
15: AO, again 70-30 against Stan (not a matchup issue for him), Wimby easy, USO.
16: AO, RG, USO. (Let's say Murray gets Wimby)
17: AO (11 Fed > 17 Fed), Wimby, USO (Weak USO, easy pickup)
18: AO, Wimby, USO (not as weak, but not strong enough)
19: AO before he's taken out of action
20: AO, USO
21: Wimby, USO
Total: 10 AOs, 2 RG, 10 Wimbledons, 11 USOs. 33 slams.
Let's say Stan wins 1 of those 3 matches against him. Let's say Murray somehow takes an AO. Let's assume he never bests Nadal at RG even though 05-08 Nadal is better than 11-14 Nadal. Then let's sprinkle in 4 random upsets. He still somehow ends up with 26 slams.
Nadal:
AO: 09
RG: 06, 07, 08, 10, 11, 12, 13, 14, 17, 18, 19, 20, 21
WIM: 07, 08, 19
USO: None.
Total: 17 slams.
Federer was very unlucky that Nadal and Novak came up when they did, because what's special about him is that he was a late bloomer and lasted extremely long. That was a recipe for just the odd win in the Djokodal era, but would be a recipe for a decade of dominance in this new post-Djokovic era. If Federer and Djokovic switched, Djokovic would be on-par or slightly below what he has now, Nadal would be slightly less than now, and Federer would be massively ahead.