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So, how will pony society change now that they have two "rulers"? Presumably Princess Celestia will still be in charge (she probably has the whole government set up to her liking after a thousand years), so Luna doesn't really have anything to do. So they'd have a rogue princess wandering around outranking everyone except her sister and doing/commanding whatever she feels like.
UNLESS Celestia wants to go back to the two-pony system, with her in charge of the day and Luna in charge of the night. It would be much more interesting and make more sense vis-a-vis Luna's mental health if they swapped or took shifts, but I doubt it'll happen. Still, we don't know how much time or effort it takes to move the sun and moon around - it seems implied that the raising the celestial object in question is the most strenuous part of it, since it was emphasized in the prologue and the festival ceremony scheduled for dawn. I doubt they have to spend all day/night doing it, since that would mean Celestia hasn't slept in a thousand years.
Celestia will probably let Luna recuperate a bit, then gently broach the subject of getting back to work. Either way, will Luna cause shake-ups in the pony government? Will there be chaos, or will Celestia use her sister to cut around the ceremony and bureaucracy that has no doubt built around her to make changes to her liking? And will Luna develop her own agenda, after - or before! - recovering from her mental problems?