Take the standard F11 setup and add a serial killer. Add x townies to counteract the additional nightkill.
Questions:
What is the best value of x?
Is the SK NK immune?
Is the SK roleblock immune?
Is the SK investigation immune?
Is the SKs kill doctor immune?
Should the SKs immunities depend on what other roles are in the setup? Thus the SK could have the same info as the mafia.
Also, I hope that x is 2 or less, so that I can mod this myself (I will be putting my name on the mini list as soon as my 3 months are up... a week and a half left!)