Maybe the path to an end of the two party monopoly in America will be the breakup of the existing political parties. For years, there have been unsuccessful attempts to start third parties in USA, but there's so much inertia in the present system that third parties don't gain traction and in presidential politics they become the spoiler. A more effective path to more parties could be the breakup of a major political party. If one party were to split, then the other party would likely split also. Today, people are afraid to break up a party for fear that the other party would remain strong and just take over. If one party breaks, that makes it easier to break the other party.
The rise of Donald Trump is certainly stressing the Republican Party establishment. Trump's rise is definitely disruptive, tho not really along the fracture lines within the Republican Party.
Bernie Sanders isn't getting as far as Trump, but his candidacy is disrupting to the establishment of the Democratic Party. This disruption does fall along some fracture lines within the Democratic Party; the fracture between the more far left "super critical of corporate America" and the Hillary Clinton style of "incremental change while working within the system." If Bernie doesn't get the nomination, I'm pretty sure he has promised not to go third party, tho. My guess is, he would reluctantly support Clinton and then, after she's safely in office, become an advocate for curbing corporate power. A split in the party could happen after the election.
In the Republican party, I see that there are serious splits, but Trump's candidacy doesn't necessarily fall along these split lines. Trump is basically just an unusual candidate so he's disruptive to the old party guard. The party is stressed at least. In the future, I see that the Republican Party could split into 3 value systems. One would be Libertarian thinking meaning "less government, less military, more personal freedom." The second would be the "religious right turn back the clock on morality crowd." The third group would be "big military, Homeland Security, corporate oligarchy, business as usual" side of the Republican Party.
Maybe third parties will happen not from new parties, but from the eventual splitting of our current parties.