But you can't "improve" the ACA into single payer. You can only get single payer by starting over from scratch.
The pathetic thing is that a public option, the original plan, could quite easily be improved into single payer. And despite the largest Democratic majorities in both the house and senate for several decades, with a sparkly new progressive Democratic president that campaigned primarly on health care reform, they couldn't pass it. It seems the Democratic party has something to learn from the Republican party on how to get shit done.
We have single-payer here in my country, and anecdotal evidence is that it works incredibly better than healthcare in the U.S. for 99% of cases (the U.S. is far better at treating complicated health problems, but at staggering costs).
But Bernie Sanders' position on health care was essentially single payer (https://berniesanders.com/medicareforall/). And he did quite well, particularly with the younger voting bloc.
From a perspective of "the left" overall, I think the seeds for single payer might be a little stronger than you think. That's especially since opinion polls seem to indicate single payer systems are fairly popular. (http://www.gallup.com/poll/191504/majority-support-idea-fed-...)
Whether enough political will can be built up to get past the perverse crony capitalism inside the health care industry, that's another question.