If you’re talking about something like an actual unified global government stronger and more centralized than the UN or something… I think that will never happen short of an alien invasion where the survival of humanity is at stake and we are truly, truly desperate. Maybe not even then.

However, there will always be an ad hoc international governance structure in the form of institutions like the UN, and the patchwork of various multiparty transnational treaties that will bind some, but not all, nations together on specific shared goals and areas of cooperation and understanding.