Very hard to predict. Most likely scenario is devolution of powers to the states

Very hard to predict. Most likely scenario is devolution of powers to the states and gradual balkanization followed over the long term by some sort of recentralization. There is too much utility in the US Federal govt over this span of territory allowing this much of a military and this much of an economy. I expect both europe and USA to head toward the same direction: federal for defense, regional for culture and trade, local for variation in culture and trade. The principle problem is culture and trade, most of the time dispute resolution whether internal (court) or external (war) is better at scale.