I think instead of outright secession what we're going to see is a blurring of state-national lines - at least in democracies. It's thus going to get harder for centralized authority of any sort to pull various stunts, and, slowly, localized regions will have more autonomy. You'll still be "American" or "Canadian" or "Australian", but the social blocks that are developing are not governed by borders. Instead they're governed by ideology and philosophy.
So I see a future where Seattle and Vancouver have more influence over each other - not just socially and culturally but also over each others' treaties, legal systems, conduct and even citizenship - than D.C. or Ottawa do, with national capitals eventually becoming more symbolic. There might be a world government of sorts, but it would function more as a body to track externalities, prevent violence and arbitrate or enforce treaties.