if u look in the tower theres a vex portal in 1 of the rooms, how about a public event where the vex invade the tower? this would only happen when a certain amount of guardians are present tho. make it a timed event as we wouldnt lose the tower but say for e.g. push the enemy back in 2 mins = gold, 3 mins = silver etc all the way down to 5 mins where u dont get anything. just an idea, it would have to be refined tho.