This season finale still leaves the question of who or what caused the  blackout?  Initially I suspected Randall, but now I doubt it since he  wasn’t knowledgeable enough to hack the Tower’s software by himself.   
Some people might suspect the president in exile at Gitmo, though it  seems (by turning on and off the lamp) it he appears didn’t have any  power at Gitmo and wasn’t much more prepared for the blackout than  anyone else. 
Besides, if the president instigated the world-wide blackout, wouldn’t  he have made preparations in advance like having the DOD fortify the  white-house, pentagon, Air Force One, military bases and other critical  government-facilities with the same type of tech that made the tower  impervious to the backout?
I wouldn’t be surprised if there’s some other yet-to-be revealed  character or people that’s responsible for hacking into Aaron’s code  controlling the nano-weapon that caused the blackout.
The US president in exile wants to “go home”, which I is assume is re-occupying and reestablishing the United States  
I’m not sure all the general populace will greet the president from  Gitmo with a warm welcome after 15-16 years, even though the rebels in  the Monroe Republic use the US flag as their emblem and talk about  restoring the US.
The president and US colony at Gitmo will need something better than  several sailing ships seen on-screen if he wants to retake the United  States.  And even if the president has planes, helicopters and modern  ships at his disposal, he’ll still need a large standing army of  hundreds-of-thousands of soldiers since occupying and holding territory  requires a lot more than just airstrikes, especially a landmass as large  as the United States.
And the president better hope the radiation from the nuclear blast that  destroyed Philly doesn’t reach what’s left of Washington DC.
It appears Tom Neville’s back to being a bad guy with the militia.  I’m  not surprised considering his alliance with Miles was uneasy to say the  least.   I wonder who will make it back to Monroe Republic territory  first, Neville or Monroe?  Even with Philly nuked, the republic still  covers a lot of territory outside the philly nuclear blast area.  
With Philly and Atlanta nuked, it opens the series for Texas to play a larger role, or as another poster said, like Jericho.
So, will the food-supply limited tribes of the Plains Nation in Kansas  and other former plain states suddenly go back to farming now that  tractors and farm machinery can power up again?   Though I suspect the  tribes won’t change their ways overnight, and the Plains Nation (and the  rest of North American for that matter) will be sort-of-like Mad Max,  with people seeking out and hoarding fuel for their now-usable cars and  other vehicles.
It’s amazing they showed of the power coming back on, though I assume  there are places lights didn’t suddenly flicker on since power lines and  utility poles would have been damaged over the years due to the weather  and other reasons.