I would favor a parliamentary system with 4-5 parties and proportional representation. I think that would help to end gridlock and stalemate and be better to represent a diverse country of over 330 million people.
Most countries with PR typically end up with a de facto two-party system, just with the two parties being coalitions instead of parties in the American sense, and those that don't often end up with a sort of state of permanent dysfunction or weirdos holding the balance of power. The US would definitely not be any different, you'd just see the Democratic and Republican parties fracture. There would be other benefits, such as the enfranchisement of people who live in strong areas for the other party, and being able to choose what faction of the party you want to support outside of a primary, and probably would on a whole be better, but it's silly to pretend that it would result in a true multi-party system, or that multi-party systems are particularly democratically healthy for that matter anyway. Look at Greece, Israel or various Eastern European countries for examples of how multi-party systems don't mean more of a healthy democratic system.
