Well most of the communist/socialists states are called "republics" or "democraties" and have elections, even North Corea. Whether or not these elections are "fair" doesn't matter. In game a one-party system would be represented by one single party being highly organised and the other being inexistent or truly unsignificant so the ruling party would be assured to keep the power.
However, there are some forms of governement in the world that _really_ never held elections. For instance, theocraty in Tibet, Absolute monarchy in Saudi Arabia etc. I wonder how those will be implemented in the game.