Universalists dream of a world empire in which a world government works to solve global problems, enforcing the same law all over the world.
There are many different ideologies that envision a world government, ranging from international socialism, to the brotherhood of Islam, to universal humanism. They squabble about what a world government would do or how it would justify its powers, but they share a dream of one world.
There are many different scenarios for how a world government would arise that truly has the power to force all other entities to abide by a single law. Some envision a collapse in ecosystems to force to a moment of enlightenment and contrition, leading to one government. Others have visions of conquest, either by arms or by divine persuasion. Others envisage a world government emerging in the aftermaths of some huge catastrophe, like a nuclear war, whereupon the survivors combine into one government. Yet others think it will emerge gradually from increased connectivity between nations and increased roles for existing international institutions.
I fear such a world government. I believe it would enslave the vast majority and lead to dreadful abuse, no matter how it arises. I have difficulties imagining it capable of remaining intact too, but that is another point.
My fear is based on the forces that disappear with the end of competing nation states.
A major force that disappears with a single governments is individuals voting with their feet, which currently forces countries to compete to attract talent and avoid losing talent. Countries do this because the individual elites in those countries benefit from having smart populations under their care. With a single world government, power is centralised and hence what matters to any local elite is their relation to the central governing system. What migrating populations do or think is then, at best, of indirect concern.
Another force that disappears is the pressure on a local elite to have the support of their population, simply because that is their power base. At present the need for that support leads the elites of countries to want to grow and to look after their populations, at least to some extent. With a centralised world system, that pressure no longer exists.
The loss of any interest in anything local is even worse at the central level, where the only pressure that remains is control of the single system and survival within it. We know from history exactly what that leads to: the centre becomes an Empire that absorbs all independent sources of power. Everything will be brought under central control. Businesses, villages, households, sports entities, culture, etc. Without the pressure to limit its power, the world will move to a Chine-style single Imperial system, but much worse because China always had some foreign pressures and its leadership could be taken over when it started to atrophy too much. When there is but one centralised government, we would get absolute control by an Imperial court.
We also know from centralised Empires what happens if some group wins absolute power and is practically without competitors: everyone gets enslaved and does the bidding of the Emperor, who becomes a god. The whole population then gets pressed into service to God. Previously that included building the Pyramids and the Terracotta armies.
Indeed, we know from human history that the move to absolutism is very rapid in a world empire: in the generation under which power is truly centralised, the deification of the leader(s) already emerges. The monstrous projects of the Chinese emperors began with the very first emperor, he of the Terracotta armies.
I have an even deeper fear, which is that enslavement might be the best future humanity can hope for. This fear comes from the realisation that in a world political system where you have lots of competing blocks that each have the weaponry to wipe out most of humanity, sooner or later most of us will get wiped out, either by accident or malice.
Would a single world empire under the rule of a human elite safeguard us from this though? I fear not. The history of empires has shown us that the fight for control simply shifts from open camps to hidden camps. Competition between the children of the emperor emerges. Different ministries compete. Army generals dream of revolt. Such tensions lead to assassinations and shadow societies at the top. When there is pressure on the whole system, due to climate disasters or a running out of food, the empire cracks and huge civil wars emerge. With devastating technology, the civil wars would be devastating, and perhaps exceedingly quick.
So I fear a world empire lead by humans and expect nothing good from it. My fleeting hope is the end of human control over world politics. A world empire lead by gods might just be workable.