I suspect one of three remnants of the Democratic Party will contain the seeds of the next political party on the left. We'll have to see what the coalescing issue will be.
The Democratic party has always been fissiparous but bound together somewhat artificially by the conviction that whatever their opponents hold must be wrong. That has devolved somewhat into the politics of reflex opposition that is the hallmark of the perennial loser, curious because for eight interminable Clinton years they weren't, at least in the executive. They are now.
I could easily envision either party or both breaking up. The Republican/Democratic party dichotomy these days cuts across several watershed issues - gun control, abortion, national defense, national sovereignty, nationalism/internationalism - that would normally represent the real dichotomy within the American body politic. It is a healthy thing that members of one party lean toward the positions of the other party on some of these but it isn't necessarily a stable one. Whatever - stability here may not be a virtue. God bless America and hang on for the ride, my friend!