I imagine a President or Shah Reza Pahlavi with a parliamentary system is the most likely outcome.
Your probably right. I bet that Reza Pahlavi would become the monarch and maybe Maryam Rajavi would become the prime minister of Iran if the Green Movement turned out to be more successful. They would both have little support as rulers outside of the northern part of Tehran and would likely slaughter any of the remaining supporters of the former Iranian government.
A Shah Reza Pahlavi would help bring in royalist military officials. Besides that, I'd guess we get a President Hussein-Ali Montazeri and a Prime Minister/Chancellor Yousef Saani.
Both Hussein-Ali Montazeri and Yousef Saani would be good choices for either Prime Minister or President of Iran and would have potentially built support for Reza Pahlavi's rule from the more traditional Iranian communities. Also, both are liberal Muslim clerics and would have supported establishing a more liberal interpretation of Islam in Iran, which might have played well with the younger generation of Iranians.