"Two scenarios for a post-Assad Syria which would be bad for Iran ..."

"... Second, while most Iranian policymakers and foreign policy elites would almost certainly prefer to see Assad remain in office, it is wrong to assume that Tehran has no options or is even a net “loser” if the current Syrian government is replaced.  A post-Assad government, if it is even minimally representative of its people, is going to pursue an independent foreign policy.  It will not be enamored of the prospect of strategic cooperation with the United States, and may be less inclined than the Assad regime (under both Bashar and his father, the late Hafiz al-Assad) to keep Syria’s southern border with Israel “stable”.  Tehran can work with that.   
Moreover, a minimally representative post-Assad government would probably entail a significant role for the Syrian Muslim Brotherhood, which has had extensive interaction with Islamist supporters of participatory politics in Turkey and other places in the Muslim world.  Syria’s Muslim Brothers take issue with the Assad government’s internal policies, not its foreign policies, especially toward Israel and the United States.  Just as the ikhwan in post-Mubarak Egypt has made clear its interest in seeing closer Egyptian-Iranian ties, the Syrian Brothers are likely to take a similar approach in a post-Assad environment.     
There are two scenarios for a post-Assad Syria which would be genuinely bad for Iranian interests.  One would be the installation of an intensely salafi, Taliban-like regime with extensive Saudi support.  But such a government would not be at all reflective of Syrian society, or even most of its Sunni community.  For that reason alone, this scenario seems unlikely absent extraordinary levels of external support for that part of the Syrian opposition which—contrary to Westerners’ derisive dismissal of official Syrian claims—consists of violent salafi extremists, see here.  
The other negative-for-Iran scenario would be the installation of U.S.-supported expatriates as Syria’s new government.  This, too, would be grossly unrepresentative of Syria’s population.  It also would almost certainly require a U.S.-led invasion of the country to effect—something that those opposition voices in Syria which have spoken to the subject have uniformly said they do not want.  Moreover, the U.S. experience in Iraq raises doubts as to whether even an invasion in force, followed by prolonged, multi-year occupation, can ultimately succeed in installing a puppet regime in today’s Middle East.  None of the Iraqi expatriates that the United States backed so handsomely—e.g., Ahmad Chalabi and Iyad Allawi—has been able to retain, by winning elections, the power initially handed to them by Paul Bremer and the U.S. military.  There is no reason to think it would be easier for America and its European and regional partners to achieve this in Syria. ..."