Great topic and definitely pertinent to many of the challenges we face as a species today.
I think that its obvious that if humanity wishes to make it to the next "step", we need to eventually come together. I don't think the division of people by imaginary lines is serving us as a whole anymore (if it ever was).
One of the problems I see with a world government is that a centralized government is intrinsically a violation of individuals as it assumes ownership over individuals. They are also easily corrupted.
I think the brightest future may hold a united world with a decentralized governmental system of some sorts where each person is sovereign (unless they choose not to be). I am not exactly sure how this would work, or what it would exactly look like. I feel like people must be free and in complete ownership of themselves in order for us to reach our highest potential. At the same time people will need an effective and efficient way to cooperate with each other.