Here's my initial draft, probably not balanced, but it's better to have a rough draft out there for people to comment (and to get this idea out of my head).
Day start.
Vanilla Mafia
Vanilla Mafia
Vanilla Werewolf
Vanilla Werewolf
Town Police (determines if someone is a Mafia)
Town Police (determines if someone is a Mafia)
Town Seeer (determines if someone is a Werewolf)
Town Seeer (determines if someone is a Werewolf)
Godfather Serial Killer (is part of both the Mafia and Werewolf scumteams, but wins individually - shows as Non-Mafia on Police inspects and Non-Werewolf on Seerer inspects)
All scum-kills are identified (so you know who the Mafia killed and who the Werewolf killed).
Victory Conditions
- Mafia wins when the Mafia team controls the daygame vote (50% + 1) and the Serial Killer is dead.
- Werewolf wins when the Werewolf team controls the daygame vote (50% + 1) and the Serial Killer is dead.
- Town wins when all scum roles (Mafia, Werewolf, and Serial Killer) are dead.
- Serial Killer wins when they are one of the last two surviving players or if all other players are dead. (Serial Killer does not need to be alive to win.)
If the last two surviving players are Vanilla Mafia and Vanilla Werewolf, then they cross-kill each other, so the Serial Killer wins.
If the last two surviving players is non-SK scum (Vanilla Mafia or Vanilla Werewolf) and a Townie (Police or Seerer), then the non-SK scum team kills the Townie player and wins.
Other Notes
The Godfather Serial Killer shows up Non-Mafia and Non-Werewolf on inspects...meaning they are indistinguishable from a Townie. This is a double-edged sword...it means the SK gets the trust of the Town Power-Roles, but it may also mean that both scumteams can easily see through the SK's facade.
Scumkills are determined by majority vote (50% + 1) within each of the scum team. If there is a tie, no mafia-kill takes place. A scum-team needs to lose one person for the other two people to know who's the SK and who's normal scum...and it also means that the SK is able to block scumkills from taking place, meaning the normal scum need to cooperate with the SK team temporarily at night, while manipulating the daygame to get rid of the traitor (which might be challenging: everyone hates the SK...but exposing your scum-affiliation in the process would hurt your chances of winning).
Power-Roles are indeed pretty powerful, at least when compared to the Vanilla Mafia/Werewolf, but since they're all inspection roles, it's possible for the scum to do a believable fake-claim and avoid suspicion. Also, the scum, as a whole, controls the daygame, so the Town is highly dependent on cross-kills and cross-lynches to help them gain ownership of the daygame vote.
EDIT: If scum kills off each other, there's a possibility that the Serial Killer could gain control over both scumkills. To prevent the SK from being overpowered though...the SK can only use one of those scumkills per night. They cannot use both.
EDIT 2: It seems that the "all other players are dead" wincon for the SK would mean they would win in a scenario where you have one Vanilla Mafia, one Vanilla Werewolf, and one SK. The SK won't vote for their own lynch, and even if Mafia and Werewolf votes to take out the third role, then they'll just crosskill each other later on...so the SK would still win.
If you had one Vanilla Mafia, one Vanilla Werewolf, and one Townie, then it's likely either the Townie or the SK would win (unless one scum-team agrees to get themselves lynched/killed so the other scum-team wins). If the scum agree to lynch the Townie, then they'll cross-kill in the night, letting the SK win... and if the scum does a No-Lynch, they will probably still cross-kill each other, but the Townie would win because he's the last surviving player (and the SK wins only if all players die).
I don't know how best to handle that - is it a feature the SK could be able to win this way? I also wonder if I should move the "all other players are dead" wincon over to the Town side, to help give them a boost of sorts (they're already reliant on cross-kills anyway to help them win).