Anything is possible, and the past is not necessarily prelude, but history is not favorable to unilateral democratization. Very few countries independently and unilaterally democratize, and stay democracies.
Most of the countries of Europe that democratized in the interwar period were strongly encouraged by the Western allies and even then most fell to internal fascism or communism. Following World War II, the U.S. put tremendous pressure on European countries to not become communist, including interference in elections. The wave of democratization that occurred in the 1990s in Europe and Latin America was precipitated by the collapse of the Soviet Union, and the resulting change in U.S. priorities for foreign policy (i.e. the Soviets were no longer around to support communists in Europe, and the U.S. pressed for democratization, where in the past it had been tolerant of dictatorships as long as they were allies). Of all of the countries to experience tumult because of the "Arab Spring" only Tunisia has actually moved in that direction, in part because of good fortune among leaders and because of a long history of involvement with France.
Iranian democratization is currently unlikely because the government has done a good job of consolidating power. In fact, even if a free and fair election could occur in Iran, the social structure of the country, the absence of a civil society independent of the clerics, and the continued strong preference of many people for a relatively conservative regime implicate that the clerics might still win. The closest analog we have is the end of the Soviet Union, which when a plebiscite was held on the dissolution of the Soviet Union, almost all regions voted to remain a part of the country!