> examples in movies or books where nations come together to face a common problem

and

> I would like to discuss realistic and possible scenarios in the coming decades where X situation leads nations to work together and society changes due to greater globalization and unification.

I don't see one. The only possibility I see is a very global huge disaster. If huge amounts of every nation are not wiped out, the ones not affected will simply use the opportunity. to move in on the others. That applies to every internal and external conflict. I love science fiction too.

Maybe a massive dose of MDMA thrown into every water supply in the world. That's literally the most practical method I can think of.