caliphate = head of state of all muslims
Shia belief
Shi'a Muslims disagree with the Sunni practice of elections. They believe that Muhammad had given many indications that he considered ˤAlī ibn Abī Talib, his cousin and son-in-law, as his divinely chosen successor making a majority vote or elections irrelevant. They say that Abū Bakr seized power by threat[citation needed] against Ali and that the 3 caliphs before ˤAlī were usurpers. ˤAlī and his descendants are believed to have been the only proper leaders, or imams regardless of Democracy and what the majority wanted, in the Shia's point of view.
Main articles: Succession to Muhammad and Shi'a
In the absence of a Caliphate headed by their Imams, some Shia believe that the system of Islamic government based on Vilayat-e Faqih, where an Islamic jurist or faqih rules Muslims, suffices. However this idea, developed by the Aytollah Khomeini and established in Iran, is not universally accepted among Shia.
Main article: Hokumat-e Islami : Velayat-e faqih (book by Khomeini)
[edit] Sunni belief
Contrary to the Shia, Sunni Muslims believe that the caliph has always been a merely temporal political ruler, appointed to rule within the bounds of Islamic law (Shariah), and not necessarily the most qualified in Islamic law. The job of adjudicating orthodoxy and Islamic law (Shariah) was left to Islamic lawyers, judiciary, or specialists individually termed as Mujtahids and collectively named the Ulema. The first four caliphs are called the Rashidun meaning the Rightly Guided Caliphs, because they are believed to have followed the Qur'an and the sunnah (example) of Muhammad in all things.
that's from wikipedia...............
to my knowledge other than terrorists showing interest in this from time to time most muslim countries have made no real attempts at organizing the many muslim states under a caliphate, its been dead since 1924............doubt all the ego's involved would go for it either, Iran would have had to project power it never had to do it by force and that too is a violation of the idea of a caliphate, and not even all the Shia support the Ayatollah's views nor do very many of the other ayatollahs what few are left
here's khomeni's book --->
Hokumat-e Islami : Velayat-e faqih (book by Khomeini) - Wikipedia, the free encyclopedia
sorry, don't see it being very realistic, uniting all muslims is a pretty tall task especially when the Sunni and Shia and other factions have some fundamental disagreements about some issues they consider important
take over a few states and make a bigger shia state, sure, it could happen, wouldn't be a caliphate though and the Sunni would not be impressed, even Al Queada wouldn't go for that. Gonna be interesting when Al Queada goes after Iran at some point, that's due to happen.