2008, 2010, 2013 - Nadal year-ending #1 all 3 years.
So if Nadal does it a 4th time, he'll be in the brink of tying Federer for year-ending #1s. Federer did this 5 times (2004, 2005, 2006, 2007, 2009).
Federer may still end up with the most weeks ranked #1, but there is a good chance Nadal or Djokovic will be ranked #1 every week for the next 4 years. Nadal will have his chance to get the weeks record or very close.
The question is, will Federer ever become GOAT of a slam event? So far he's sharing the lead at AO, USO, Wimbledon. Nadal however is already GOAT of Roland Garros, with a 2 slam lead over Borg (8 Nadal, 6 Borg) and continuing to rule. Also keep an eye on Nadal at US Open. Nadal has won it twice, is now improving on hardcourts, and only Djokovic seems likely to threaten him each year there. In Nadals 2 US Open crowns, he was only broken 4 times in 2013, and 5 times in 2010. 4 times broken is the outright record (and 5 times broken, tied Roddick). As for number of US Open titles, Nadal has an excellent chance at 4 US Open titles, and an outside chance at 5 US Open titles (that would tie the most).
The fact that Nadal is the only man ever to win slams on clay, grass and hardcourt in a calendar year is beyond HUGE, however I'll let Federer slide on this, because Federer has multiple years of winning 3 slam titles (even if it was the hard-grass-hard combo each time).
But I won't let slide the fact that Nadal has won slams for 9 years in a row, and only getting stronger on hardcourt (the surface of 2 slam events each year). Nadal has that record outright already, and will likely extend the gap.
And then there is the 26 Master shields Nadal has won, very likely to be 30-40 in future. Federer has "just" 21. I don't think World Tour Finals titles will be able to make that up for Federer, especially considering he's already lacking a singles Gold (and as I said earlier, many already value singles Gold more than the World Tour Finals). And before you point out the doubles Gold, we all know that that is a doubles achievement (it belongs in a comparison with Woodies or the Bryan Brothers).