If you ask anyone in the world to do something for the sake of Allah where there is no benefit to them (and potentially even material harm) maybe one in a hundred will. The rest will look after number 1, and this is natural.
The leaders of countries, especially in this era are the least likely people to do something for nothing. They are power hungry people, the exact opposite of what an islamic leader should be.
Imam Abu Hanifa (ra) spent his final days in prison. He was imprisoned because he refused to become the chief judge. He told the sultan he was not the right person, the sultan said he was lying, to which he replied even more reason not to appoint him. Men of that calibre deserve leadership, not those who seek it. Knowing this we know the benefits of Islam cannot be used to reason with our current leadership.
In light of this any union of Muslim nations would initially need to be built on worldly benefit - of which there are many. I suggest anyone interested in seeking a union of Muslim nations study the formation of the EU.
A few things to consider;
1. Phased union. The EU didn't happen overnight, it took the best part of 40 years to get here.
2. Economic parity (there must be financial benefit for all. Participants must close the economic gap before joining).
3. Limited migration between countries.
4. Keeping national autonomy. Until the union is strong and well integrated nation states should keep political autonomy.
5. Joint interest. All participating nations have to have joint interests.
Slowly as the economic imbalance is balanced out (and percieved threat to finances is reduced), Migration will be easier, laws will be harmonised (as they are in the EU), policy will be built and implemented at a union level, the political leader will gain more power, foreign policy will align, defence resources will be shared, but it could 100 years. In that time if education levels increase and people dedicated to the cause of the caliphate propagate the message, anything is possible.
