You're right, the reason why I said that Charlemagne was the head of the church is because he was made the de-facto ruler of Italy, a nation which he had conquered but had many problems with over his tenure, through being crowned by Leo III. As a sort of continued line of Roman Emperors, but this was actually very smart on the part of Leo, because by crowning Charlemagne he effectively ensured that the papacy would have the right to crown whoever they choose as Emperor, in actuality giving them an advantage over Charlemagne. This ensured the union between Church and Emperor. But you're right. History tells us that in monarchy you must have a successor, and history is full of great leaders who like you said did not plan for the future. In terms of politics and the change of culture Constantine the Great is definitely up there, something I will do more research on. Charlemagne is more remembered, I believe, for his military achievements. He was not a politician. Great rulers need to be warriors and politicians. History is great though.Yes, his career was quite an achievement. But he was never head of the church. The pope was the head. He got the title of emperor and all the glory and honor that went with it, an important political tool, in exchange for spreading the word and defending the church borders.A very nice deal for him but at the time Rome really needed a strong warriors arm. Just like Alexander he was an excellent commander and just like Alexander he didn't have much of a longterm plan for the succession. Just like Alexanders Generals split up his empire in several parts and started to quarrel and fight eachother so did Charlemagnes sons split up his territory and start to fight eachother. If not then we might not have had the division between Germany and France that resulted. The title of emperor could not be divided so it went to the son that ruled what would in the course of time be germany and was soon called the Holy Roman Empire. That many emperors there saw themselfes as head of the church with the right to decide over the pope was a view that no pope ever shared, at least while the seat was in Rome. The Avignon Popes was another matter, they were seated on french soil,were of french origin and followed the hints of the french Kings. Charlemagne was a flash in history, a bright one, but since he didn't plan for the future much of his influence is not as great as it could have been. He made the same mistake other kings has done before him in splitting his kingdom in eqaul parts among his sons in the vain hope that they could coexist without quarrel. He was wrong.
An example of a person that was both head of state and, if not head at least influenced it very much was Constantin the Great, Emperor between 305-337. He united a divided Roman empire, Took christianity from one of many religions to be the religion of an Empire, founded Constantinople and forced the divided Christian bishops to agree on what Christianity was at the Church Council of Niceae 325 which he attended and was chairman of. His utopia of an united Christianity didn't realize neither then or now and is even more divided now but he was probably right in arguing that an empire needed a firm religious ground to build on.