Most near future space faring science fiction makes one almost constant rock solid assumed trope: Human is ruled by one world government or one super powerful nation has almost complete control. Always earth vs colonies or earth vs belters or even earth vs mars. Its not a bad trope really its, pretty decent assumption that in order for humanity to have enough combine resources fuel our escape of our home for something else it would require almost all of our attention: a goal for all of humanity to strive for.
Except given how technological development goes and how goods like cell phones go from Military industrial complex, to high end expense item to cheap come good that the just in the past decade went from the minority to the majority of humanity now has cellphones. This was not done because of good will or a great goal. No, it was done in the name of profit. The success of free markets is their ability to take expensive goods and rapid make them inexpensive enough for the majority of the market now able to buy it. R&D of next generation of tech can be also strongly debated that corporations and individuals are much more capable of making right steps than the governmental agencys like NASA. Given now that Space X is basically redesigned from the ground up how to launch humans into space, though we have yet to see how cost effective that will be. The point is there strong if not stronger argument that humans will get to space in mass not by uniting humanity but by the greed or pursuit profit by corporations. The colonization of north america is good suggestion that it is possible.
Most modern Americans would be surprised to know a good chunk of the early British colonies in New England were charter corporations even some of the early states were considered as much. Now East India trading company they were not but there formation was the same idea: Potential high ROI. Venture Capitalist funded James town expedition in part for the hope of gold in return if discovered in the new world. Now gold was not found but something just as valuable was: LAND and lots of it. This defiantly a simplification of the early american colony era but the major economic driver bring people to the new world was old world charters corporations in pursuit of profits then after that new opportunities because land was cheap but long term it was gold.
So what is the gold of the stars? Rare earth minerals. Currently a there is decent theory in geology that holds a good chunk of all metals partially rare earths came from the sky in the form meteors, meaning if true, probably a good chunk of those rare earth rocks we need for current/future tech is in the asteroid belt, right next door, and those most likely want those would be tech corps because the profit potential would be insane if you had practically unlimited amount of rare earths metals and easy to mine common transitional elements as well.
So where does the multinational theory come then? Basically the reason to unity the world never happens because it Corporations would cheapen the cost of space travel so much in pursuit of the rare earths unity humanity under one goal is a non viable idea. Sadly most sci fi writers I feel do not understand a most humanity does not think we are one race let alone they forget there good deal of science that humanity is not all the same even genetically,physically and mentally and this even before cultural problems of some groups never will be compatible others in a reasonable time frame but probably given history more likely to be subjugated or conquer instead making relations with the other cultural tribes of humans.
So its very likely the future space faring world would be just like how the world is currently divided between three super powers: USA, China and Russia and up incoming powers like India, Brazil. The distrust of each will lead to another space race though this time it will be waged via corporations for the cheap resources of the belt and that the nationalize is not a disease as globalists would think but is a defense mechanize of humans, IE a carry over from our tribal times, to defend against out side threats. We are smart monkeys but still very much monkeys,we forget that we decide more on what is best for the group of people we know than might be good for all of the monkeys. Would you save an average stranger or your close friend you knew from your tribe for most of your life? My bet is on the latter for most humanity.
To sum it up its far more likely we get massive drops in expense of space travel from profit motivate corporations than from a theoretical global government, Humanity never will like unite given no common enemy(the Sci Fi Visual Novel Muv Luv proposes we still wouldn’t even if we did have one.), plus cultural, historical and our tribal bias would prevent that from happening as well.
The implications of this is we might ended up in a future where humanity on earth might be at some sort of cease fire between countries but cold wars and fringe fights over deep space territory would happen if cheap FTL came about but the true rulers of earth would be the massive interstellar corps that profit and manipulate the earth governments to protect them from the other corps and many steal territory for me for them. Earth is no longer worth ruining and fighting over if you can get it all done in the shadows of deep space.
The UN would be useful for one thing only: enforcing the rules that might hurt most of the earth powers and pretending to be something noble while most of planet knows its just part of the act. The real potential in this setting is that each nation and corporations can be played against each other and darker side interests would be playing forces against each other let alone not to mention I forgot to talk about the potential mercantilism that would be put on the colonies to befit earth/SOL because potential threat some advanced tech. Say the Dangers of Grey Goo scenario due a real scare only allows earth corps to make nanites anyone else is consider a threat against humanity and hence they lose any advantage that tech might provide let alone divide humanity not only nation vs nation but earth vs the colonies.
This just a part of my line of thinking when I was creating the SP Universe. Feel free reply.