A kind of union where the national interest of Indonesia is not different to the national interest of Algeria or Pakistan.
Muslim countries need to be controlled by a central authority figure - that is the Khalifa. You gave the example of the Governor of Syria having his own army - that caused fitna.
All economic and military power, all political authority needs to be central. The USA is an example - each state has it's governor, but the federation is where the real power lies, with the President.
We will not see it until we have people in power in each Muslim country who fear Allah more than they love power.
I think we can transition to this in an EU style model.
Unlikely to happen. Dont translate the word Caliphate literally, in essence it has no different meaning with international organization like OIC or NATO for Western nation.
The leadership should be naturally formed based on Muslim leaders perception, just like how EU or NATO are governed. In the time of huge challenge, OIC will likely be more effective like EU and NATO with real Russia threat.
Centralistic governing is not ideal, it is likelt to create dissastifaction and inefficient system to respond on various and different needs of each Muslim nation and people. This is why even in the nation level like Indonesia and many other nations we prefer to have provinces and cities with greater power in governing their own areas, budget, choosing their own leader, local law, etc.
Current situation is already ideal. United Muslim nation will create ineffective government and will create huge conflict and dissastifaction from for instant to answer this question like where is the capital ? Who should be the leader ?
Madinah Charter made by prophet Muhammad is more fitt into the Nation state as both Muslim and Non Muslim within the Caliphate are accepted as the citizen and will be protected by the state.
Under current and even since centuries ago, the Muslim brotherhood is played in the form of cooperation between Muslim nations. This is why Indonesian ancient Kingdom get help from Ottoman in Turkey when they were against Western colonial powers. When the challange is so great it can force Muslim nations to make some kind of extraordinary cooperation between them like how NATO operates. OIC as organization has capability to do that, as for now why we need to create such as extraordinary cooperation as we dont need it