Maybe it could be something similar to the idea of houses in Hogwarts, where a group of people have found a way to harness mana, causing some sort of divide in the school?
You could have the main conflict being that the mana is depleting! See where that takes you! Or that people's mana valves are getting smaller? :) I'd love to help you work on a plot further, do you yourself have any ideas so far that we can build upon?