Governing affects things locally, so you always need some kind of local representation. In the USA we have the states, united under one federal government. It works... however, if you're stretching to distant lands in different time zones and significantly different regional cultures, it does NOT work (the USA does this in a very limited sense, e.g. Alaska, Hawaii). Human social history is a good indication of this, as empires sought to dominate and control a significant portion of the globe, but ultimately ended up shrinking back to more "manageable" proportions.
I see an eventual UGC of some sort (United Global Council) formed to address issues that have global importance. This is becoming increasingly imperative, as we've no global accountability at this point. Rain forests, seas, and wilderness stretches (to name a few) are being abused at the expense of global security. The unbridled exploitation needs to stop, or we're going to ruin the planet for comfortable and sustainable human living. You can't effectively limit exploitation without a global entity of some sort, because without it there is no way to enforce accountability.
So while I don't see a "world government", I do see the need for something like the United Nations that has a much more active and effective role in global affairs.