I think the problem for a democrat-communist USSR is not communism, it's Russia. Most of Russians have no idea of how a democracy works, Russia as we know it is a huge Moscovia, and the only way for Ubermoscovia to keep it's lands together is to be despotic. When Catherine the Great summoned an Assembly of free people (1/10 of Russia was represented) they didn't do anything but find her a honorary title "The Great" ... and they were the educated 1/10.
The problem is that if Russians don't know how to handle a democracy before a bloodbath, they won't be more experienced after. The same thing happened with French Revolution.
So my pick would be a Constitutionnal monarchy in Russia, more and more democratic, and then a Tsar that we'll call Ivan Stupidovitch starts being a horrible ruler and gets kicked out of the throne
I'd like to defend the large estates general (in essence) that Catherine summoned. They DID have suggestions. They had suggestions by the dozen, on all sorts of issues from the highest levels of society to the lowest. If she had kept them meeting and given them actual power to draft and pass legislation, I think they would have come up with some very interesting stuff.
As it is, though, Pugachev's rebellion cut it short, and after that she decided to abandon her reform plans and just rule the country as a tyrant. Honestly, it's one of the things I find most disappointing about Catherine as a person. She was *this close* to building an institution that could have changed Russian history for the better, and she strangled it in the crib because of a completely unrelated uprising.
That's pretty much the story of Russian democracy/republicanism every time it tries to get off the ground. The rulership gets pissed because people who have never done this before aren't very good at it, and rather than giving them time to learn, calls the whole thing off.