How about a Random or Rotating seer?
This would be to give all of the villagers the possibility of sometimes having a night action.
Random Seer: Each night, a villager is randomly selected to be the seer, and get the use the seer power. If the appointed seer is killed that night, no more seer power for the rest of the game.
Rotating Seer: The seer is randomly appointed for night 0. When the seer investigates someone, they are told whether or not their selection is a vampire/werewolf or a villager. If the selection was a villager, then that person will be the seer for the next night.