I think instead of all religions joining as one, there will be little religion. This would allow more unity, if a large degree of the world is not neccesarily aethist, but rather more neautral towards one another with a general belief of God, and all else goes. The different beliefs are a bit complex to join into one super religion that accounts for them all. It's much easier to just create a religion of "self", and as most would say, it's the easiest religion to follow. Generally, the entire world would have to be much more globalized at this point. Take Canada for an example, it's a very multicultural country, and generally, there is acceptance for everyone. Now imagine that mentality across the entire world, alongside very little religion.

Is it possible that unions will replace governments(for example, an Asian union, North American, European) in the next 100 years? I'd say the chances are good, but not for the whole world, select nations at first. Of course, things can move quicker or slower than meets the eye. One has to ask, when will the madness stop, will there eventually be one currency for the entire planet? Will the final unions merge 200 years from now? What will the Playstation19 be like?