How will the major parties be replaced? Which parties will replace them?

[Skill and Chance]

Not happening absent a second Civil War or a war with a foreign power that goes so poorly that there is an invasion of the continental US.

[Skill and Chance]

You mean if Democrats can't win or if Trump really blows up the GOP?
The most likely scenario, based on the last few cycles is that there will be a major problem materialize and that the Democrats will find a campaign that can repackage their platform. They will then be in power and either pass their agenda or not and quickly be swept out of power as conditions improve and the Republicans put up some one who is progressively less conservative and more of a right-wing populist. The Democrats struggle with the "realignment" until they repackage themselves and either wait for another crisis or push sufficiently to the middle.

Trumpism leading to another 1912 down the line is plausible.  It would probably be in 2024 if the next GOP nominee is insufficiently Trumpy.  But look how quickly the GOP came back from that last time.  Wilson won by one state in 1916 and then Harding broke 60% in 1920.