I see the next major phase in the political evolution of the United States as either the collapse of the Democrat party, or its schism into a more moderate Green party and a more radical party which claims the mantle of the Democrats.
The Greens have found some success in Europe by focusing on only environmental issues, and being flexible as far as the rest of policy issues go. They please their constituency with their neutrality. And over time it has pushed them to become more moderate and results oriented rather than Moonbat.
In turn, the Republicans strongly need to purge some of its liberal members, which might form a Centrist party with more moderate Democrats.
For an election or two there might be more than three major contenders for office, but it will probably settle down to three for a while.
Third parties won’t take under the Electoral College and the representational structure of Congress. Only under a parliamentary system could the Greens get any real power.