I can't really decide, but, in terms of which major is most likely to be one by someone outsider the big 3, the US Open is the one I would expect.
Australian Open lets the big 3 come in fresh, and Djokovic is going to be the favourite going in for the next few years.
Nadal remains king of clay for the next few years. If he's ever too worn down by the spring hard court season, Thiem will have an shot here, but I think Rafa will do everything he needs to do to remain healthy for this tournament.
By the point of the grass season, the players should be getting worn down, but, the size of the stage in Wimbledon, along with being such a unique surface,it favours the veterans.
US Open next year though seems like it's got a shot for a breakthrough. It's at a pretty strenuous part of the season, especially being an Olympic year.
If I had to guess a name, Medvedev seems the most obvious, even if I know I'm probably letting recency bias impact my decision. But, I think someone like Shapovalov could bring his game back up by this point next year and be a threat.