Should there be? Well that would certainly make running this planet a lot easier, no more international relations between independent actors that could always possibly lead to war or other conflict. But is there any possible constitution that could be made today that would do any degree of a good job at that? I think no.
Let's take the founding of the US as an example. 13 states that joined together to create one unified nation. A high majority of the people in the states were former British subjects, thus they were all very similar in culture, race, language and many other factors. They had an easy time(comparatively) of unifying, if not for any other factor than that they were essentially already the same people.
Now compare that to the present day where most nations are pretty spectacularly different. There are Europeans who don't want anything to do with American health care policies. Russia and Uruguay have very minor relations with each other and would like in no way to be politically bound to each other as it would only be mere inconvenience to them. There are even more extreme examples such as Islamic(or other religions) extremists who can't even abide the existence of non-believers let alone joining with them in political ties.
Another essential unifying factor(in any country/world) is the existence of a great threat to their protection and safety. The US had to scrap plans for the Articles of Confederation because there was simply no way to deal with the threats that came from internal rebellions and the threat of invasion from Britain. So the greater threat of these things to our survival caused us to unite under much stronger bonds in the current constitution. I see of no significant threat that would encourage the world to give up the vast and complicated status quo in order to form a higher governing body, let alone any agreement on what and how that body should function. Some plausible ones for the future might be a world water shortage, global warming disaster, or more extremely, an alien invasion, but we won't be seeing those for a while and even they may not be scary enough to start any urgent unification.
Some scientists have proposed a more optimistic view of worldly unification where, before we venture out in to the stars, humanity will have been enlightened enough and unified by the search for scientific knowledge. It's a nice theory but there is no real historical evidence to support or not support this.
One possibility I have always entertained was the unification of the fifty states, the UK states, the provinces of Canada, Australia, and New Zealand and the territories of those nation under one Federal style constitution. We are a much more homogenized group of nations, having all stemmed from similar sources and I think that has potential for great success. But that's a mere fantasy of mine that I thought I would share. I hope this post helped!