From my Pinot of view the supreme leader position should be political only white the wali al faqih should be a religious position just like the pope
that was exactly what shapour bakhtiar (interim PM in the revolutions transition) wanted. he wanted khomenei and all the mullahs to go to qom, and serve a position similar to the pope. And pretty have them create an Islamic utopia there.
This view was/is violently rejected by the government. And its considered treason to even consider it. Its actually the position taken by many reformers/ and even some anti-government people who are not seeking the complete dismantlement of the current structure.
as far as the next supreme leader is concerned. If the government is smart they would change the position to a kind of council, with a semi-permenant/ rotating head. (something similar to china)
this strengthens the government enormously. Khamenei has built an enormous support base around himself that stretches into every sector of government. Any important department has a supreme leader representative who de-facto is more powerful then the head of the department.
Khamenei also has a semi-cult status among his supporters that would be hard to carry over immidiatly into another supreme leader (at least in the short term)
I cannot see how some mullah like a larijani can step in and have legitimacy among anybody. This would cause severe instability.