Mass Effect actually had one of the more realistic representations of a future humanity. All of the nations today still existed, but when we started colonizing space the major nations formed the Systems Alliance. Each nation paid into the systems alliance so it could build ships and handle administration of off-world colonies. How much influence a nation had in the Systems Alliance was determined by how much money and resources they put into it. So, naturally (as indicated by the codex), the United States, Russia/China and the EU were the real influence behind the SA. At first, the SA had no real governing body whatsoever except for colonial administration.
When the First Contact war happened, the individual nations had trouble organizing their personal forces for defense and it was the Systems Alliance military that ended up winning the major battles. After that, the Systems Alliance became more of a global European Union, gaining more authority, a legislative body, and the ability to dictate laws that must be followed in the colonies. Obviously, the makeup of the legislative body and SA leadership was still effected largely by the nations who contributed the most (US, EU, etc).
There is one of two possibilities i see realistically for humanity in the future. Forming something similar to the Systems Alliance, or each major nation colonizing space on its own with loosely followed treaties and a shaky alliance preventing an interstellar war between human factions.