The Investigator is either one of the most underpowered investigation roles available, or with detect exact roles on the most overpowered role.
Why is it so underpowered? It's ability is detecting clues, for clues to be available a role has to have activated the clue by using an ability.
To improve it I suggest adding more clues, not just relating to the role but to what roles have been visited by.
Here are a few:
Bus Ticket: Been driven
Blood: Been involved in a fight.
Courted?: Been roleblocked
Gasoline: Been Doused
Poison: Been poisoned
Magic Aura: Been witched/oricaled.
The idea is for Investigator to become a feedback checker, to see who hasn't claimed feedback or who is lying about what feedback they received. Also to detect what roles are in the games.