I'm not sure that it's highly probable or probable at all.
To get there the first prerequisite is survival of the species and the planet if it is to happen on Earth. There is climate change, the threat of nuclear war still, chemical warfare, natural disease from population density. And I'm not even talking about supervolcanoes and asteroids. So first thing is to survive.
Next is the fact that we still have segregation and conflict within nations, within states, within communities, families and organizations of all kinds. So if separation and conflict is rife on the smallest scale of the collective, it seems to me unlikely that unity on the biggest scale there is will happen successfully.
Lastly there is the question of the meaning of it. A nation is a territory, a constitution, a demographic and collective of provinces or states and cities and communities. We vote, pay taxes, enjoy public services, civil rights, commercial, residential, recreational and educational infrastructure. But this is just a contrived system created to organize efforts, beliefs, cultures and resources towards common well being. So say we get to the point were the cultures, beliefs, efforts and resources of the world are organized together for the common well being of all involved.
Well in a way we're already doing that to some extent. But even on the grand scale of One World it is merely the realization of efficient socioeconomics and politics. Its a nice goal but not the ultimate potential of life and humanity.
So I've shown my perspective of this as being merely a tool for organized well being and also that it does not seem probable with the way things are going. What this can only mean is that humanity is in a more dire state than it seems. We have an administrative goal so to speak of organizing the world into One World, and much more important goals such as exploration of the galaxy, discovery in physics and mathematics, understanding and utilizing the mechanics and chemistry of life (I don't mean horticulture), spiritual exploration (discovery in subjective consciousness) and just overall happiness and abundance of life in general.
We are like a lazy A student playing truant to do drugs and and gang-bang, nearing the end of his/her school year facing expulsion and a life of poverty if lucky or death by murder, overdose or cancer if unlucky, but with the potential to cure cancer, lead the nation to peace and prosperity and save the world.