The GM:
My original idea was "Become increasingly powerful"
But... what a good idea to have a bigger objective...!
I'm open to suggestions...
All of my ideas, unfortunately, involve more work for the GM. For instance: with some of the successes, we recover information that we, the players, have to put together to reveal the existence of other missions. Eventually, we have missions coming from you, and missions coming from the clues (assuming we figure out the clues).
All of this builds, leading to an ultimate mission to deal with some larger threat that may require all of our combined forces. Perhaps the final elimination of the Cult of Callum, or the thwarting of its effort to bring the Demon of Annihilation from the Pits of Hell -- or something like that.