I'd set up a fairly limited constitution to safeguard a fairly small set of fundamental rights and bind the hands of the one-world government. I'd establish subordinate governing bodies ("states") along roughly the same lines as our current nations, respecting historical and cultural boundaries. The one-world government would adopt a federalist posture and let the states govern themselves for the most part.

One fundamental right I'd guarantee is the right to free movement. No state could prevent its citizens from emigrating or stop other states' citizens from immigrating. Preventing states from holding their citizenry hostage would greatly reduce their ability to commit civil rights abuses. For instance, how many people would be left in North Korea right now if they could cross into China or South Korea without being shot? Not many.

The more affluent (and hence mobile) a citizenry becomes the less able an individual state government is to screw them over. Having a global "lingua franca" would further advance the goal of making people mobile (and thereby insulated from state abuse). I'm biased, but I'd nominate English. It only has about half the total speakers as Mandarin, but it's spoken in a much more diverse set of countries. Plus it's character-based instead of symbolic and is already cemented as the language of international business and science. So I'd mandate (somehow) that all states incorporate ESL education with the goal of basic English fluency.

One of the primary obstacles to this plan would be single nations with large militaries. We could dodge this in a couple ways. First, we could split up the "big military" states into multiple smaller ones. For instance, the U.S. and China. You have to figure the pieces of the former U.S. would still be fairly culturally unified, though, and willing to band together. So breaking up the U.S. wouldn't be enough by itself. Instead, you'd need to cap the size/strength of state militias (since, in theory, they'd be unncessary). The capped level would need to limit them to a size that's smaller than the global "peacekeeping force" that would be comprised of soldiers from every state. The global peacekeeping force would be the "might" to back up the rulings of some sort of one-world "supreme court".

The one-world supreme court would be in charge of adjudicating the sorts of nation-against-nation disagreements that, today, often lead to war. For instance, the border dispute between India and Pakistan.

Nukes represent a big problem too since they greatly empower any state that has them. So we'd have to have world-wide nuclear disarmament and a global ban on the development of nuclear weapons.

One positive is that, theoretically, global military expenditures would be much lower, which would free up a big chunk of productivity to other pursuits that are more likely to enhance humanity's well-being. Defense spending is basically "overhead".