From a July report by Barry Rubin of The Johns Hopkins Foreign Policy Institute:

Iran will face a significant but not unsolvable leadership conflict after Khomeini dies or is disabled. It seems quite unlikely that Montazeri and Rafsanjani will agree to a division of power, which will be precarious but workable. Many Iranian leaders will jump on Montazeri's bandwagon. If he can govern with reasonable competence, Montazeri should be able to forestall any serious internal challenge.

. . . The two scenarios that might make for a drastically unsettled future involve Iranian aggression against Gulf Cooperation Council (GCC) states or Soviet pressure on Iran. However, Soviet intervention is unlikely except under dramatically altered circumstances -- most likely, sufficient internal turmoil that prompts an Iranian Islamic faction, by no means necessarily a more "leftist" one, to ask for Soviet assistance. Soviet intervention might also come in the form of a major covert competition with the United States for influence in Iran, which could lead to a Soviet military presence or direct superpower conflict in Iran.

Again, it should be stressed that this is relatively unlikely. . . .

A more likely problem would be Iran's decision to prove its primacy in the gulf by threatening or attacking members of the GCC. Most probably, this could come in the form of an accidental or deliberate "spillover" of the Iran-Iraq War into Kuwait in the aftermath of an Iranian victory or as a result of Tehran's frustration at not being able to win the war otherwise. In such a situation Iran will be most likely to bully GCC states and pressure them toward various specific ends -- that is, to reduce their ties with the United States, to abandon Iraq, or to follow Iran's lead on OPEC policy.