Olympics 2020 is my pick, but there's very few opportunities on Nadal's calendar for this to manifest
Australian: Want to believe it can happen (against Federer, not against Djokovic certainly), but history says it will end in bitter disappointment
Acapulco: Djokovic played once a few years back, lost to Kyrgios as I remember, but not likely to see Djokovic/Federer participate
Indian Wells: One of the rare tournaments where each has a history of multiple successes (3+ titles), but coming off the Australian summer (ATP Cup + Oz), exhibitions (Federer, Dimitrov, Ferrer), etc...I think/hope this is where Rafa begins to rest the body for his annual clay assault, so don't think he will progress to SF/F where he would meet Roger or Novak
(He's not playing/should never play Miami ever again)
Tokyo Olympics: Federer and Djokovic are gunning for this as it's the last chance to add Singles Gold to their respective accolades, but Rafael Nadal, playing for Spain, and the rigors of the calendar are going to be factors (how he found a level to compete in Rio out of the 2015/2016 doldrums idk) and this is the spot where it most likely happens (would trade for a major hardcourt final in a second haha)
Montreal/Toronto: The Big 3 have started to coalesce around one Masters 1000 as sufficient preparation for the USO, and I especially see this holding during an Olympic year...Rafa has clearly opted for Canada and Djokovic/Federer for Cincy, so similar situation to Acapulco
US Open: In my estimation, Federer is done being a factor in New York...the humid conditions and the rigors of the tennis calendar are too much at his age, especially coming off his primary efforts at Wimbledon/Tokyo Olympics...Djokovic and Nadal can happen, but I definitely back one/both to not make it to SF/F based on the tennis calendar coupled with their age, there's a reason most tennis fans/"experts" believe this is the next slam that the Big 3 fails to secure
Shanghai: Rafa skipped the entire Asian swing last year and hope he will adopt this going forward, but we all know he'll play Shanghai if fit...the speed/surface conditions don't aid his game, needless to say, it's not happening here
Paris: Federer has passed on this M1000 repeatedly to give his body ample rest before WTF and after Laver Cup/Basel...again Djokovic and Nadal can come to pass, but as we saw last year, Rafa is likely to WD or suffer an early exit as the indoor conditions don't aid his game
London WTF: Last year was Rafa's best chance to win this (in London) and this is where it's least likely to happen in my estimation...I'm ready to see how Italy plays lol