The answer is breaking the duopoly. Unfortunately they both know that, and that once upon a time they were 3rd parties, and they are completely allied in keeping that door well locked.
Our system - executive system is ill-designed for 3rd parties, parliamentary system is optimally designed for it. In our system 3rd parties usually coalesce around a single to a few issues. Those issues later get adopted by the major parties or die out.
The Founding Fathers purposely designed our system to minimize the number of “factions i.e., parties”. The minimal number and not be a one-party dictatorship is two. In parliamentary systems there is no real executive authority. The majority party or coalition picks all the executive\ministerial\management positions. Everything is subject to party (or coalition) politics. Our executive system doesn’t have as strong or direct enough “political award system” for co-governing to make coalition politics work. You think partisanship is bad in our system, in a parliamentary system it much much worse! You need a very strong “independent (as much as it can be!)” civil service to have any continuity of government. You think our Deep State is bad its nothing compared to the civil services of say the UK, France, Japan, Italy, Germany, etc. In France they have their own ‘bureaucrat MITs’ to train government bureaucrats. In the view of these bureaucrats, they run the government. Politicians come and go and are more an irritating distraction or entertainment then anything else.
As bad as our system is, everything else is much much worse!