Uninationalism

From the few books I have read(there are likely to be many more with similar concepts), I can already name an example where this system/idea has been addressed, in the Bean quartet by Orson Scott Card, specifically the last novel. In the book, Peter, as the Hegemon, establishes the Free People of Earth, which is essentially the idea you're addressing. This worked relatively well (you can read it yourself or read a summary). However, it was achieved through war and military prowess to subdue the government and let the people join, which if applied in today's world, might lead to everyone bombing each other if anyone tries it. To keep peace after the FPE was established, they consolidated the armies of the nations of the world and reduced the power of a nation to "what state means in America. An administrative unit, but little more." This eventually fails, due to the corruption and selfishness that plague humans. About the alien thing: in the same series, the immediate threat of an alien species united Earth, but as soon as it was gone, the nations went right back to fighting each other.

Summary: Almost just like communism, this idea is great on paper, but fails because we are who we are. Maybe we can accomplish this temporarily with a great leader, but it won't last.