I just read this question which asks "What kind of government is suggested by Islam? Democracy or Kingdom or …?"
However, I thought it seems a little too broad or opinion-based at such level, because it already assumes (or have to assume) that Islam as a religion is inclusive of a theory of government. However the assumption I believe can be supported as a valid idea.
First, because Islam is considered by Muslims to be the source of ultimate guidance on every aspect of life. This indeed can be understood from various Quranic verses, the most explicit being the verse which asserts: "We sent down you a book that is an explanation for everything."
On the other hand, we also know that Prophet Muhammad himself established a government in Medina which he ruled and governed according to teachings of Islam with collaboration of his companions and followers. In fact, some historians of political theory argue that the formal agreement the Holy Prophet signed with tribes of Medina (known as دستور المدینة or Charter of Medina), can be regarded as the first formal Constitution in history. (See the wiki-page, Constitution of Medina).
Therefore, there does seem to be an actual doctrinal and historical basis for a government/political theory in Islam.
So the question is: what are the other doctrinal or historical evidences that can be used for elaborating a theory of government according to the teachings of Islam. And what are the existing theories of Islamic government, if any, as formulated by Islamic scholars?
Post-script: Please note that this question is not asking for opinions but
1) Quranic and narrated statements related to politics of Islam which may help us in postulating an Islamic political theory, and
2) existing theories on Islamic government.