A one-world government is a logical end point for democratic nations; dictatorships and monarchies would likely have to join the modern world for a OWG to function. I could see it working for managing trade, disaster relief and mediation of disputes between nations, but standardized legal and political systems among nations will likely never occur apart from participating in the selection of representatives in the OWG.
Optimistically, it would probably function as the US aspires to be; States (in this case, entire nations) functioning autonomously (sovereign) wherever possible, but with the OWG capable of setting the terms in certain areas. This would probably be the most attractive model; it has the appeal of both participating in global cooperation (ensuring maximum influence) while retaining a national political identity.
