In theory, it certainly is possible to envisage a United States of the World.

Constitutions from many countries are very similar to each other as they get inspired from one another.

Federal states already exist and enable many issues to be dealt with locally, rather than nationally or internationally.

It's mainly a matter of competence. Who's competent for what? A coutry (like Belgium) can gradually change from a centralized one to a federal one.
If everybody agrees on federalism and there are states or departments or regions that have the same population or size, the matter becomes even easier.

Raoul