Why the change?
Most everyone has a grasp over this concept, but I think it's high time we eliminate the doctor from our standard roles. Being able to protect indefinitely with impunity is so broken it's dumb. Find the Doctor before the other power roles destroy you becomes a huge part of a scumteam's game.
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If you're not going to change the Doctor role in your setup, you need to:
A) Weaken the Town's power.
B) Properly equip the scum to combat the combo.
If you don't want to give the Mafia a blocker or some kind of investigative manipulation, how do you justify including protection and [insert power here] on the town side? The Jailer is one answer, but if you don't want to give Town a blocker of its own, the restricted Doctor role is a great alternative.
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The variant I have played with a lot recently is such:
This creates a lot more information for the Town than may be necessary for a game, and can still lead to chain-protection so instead we can neuter it down a bit:
Anyhow, thoughts. Yes, it's a bit of a hard line, mainly to sell the copies, but I do think the Doctor is a root of most balancing evils.
EDIT: Forgot to address the self-protect part of the new role. This allows a bit more flexibility to the setup balancing act, and also creates some lovely WIFOM minigames at Night, which we all love. Well, maybe.