Regarding roles, I'm sure you're aware of my preferences toward open formats and against putting pro-Town players into the Scum QT - particularly when the latter is done unannounced.

A Spy role which appears to be converted but instead remains pro-Town, or one which does that but also learns the identity of 1-2 Cult members seems fine. But if you just tell them the identities of everyone who's a cultist, they're just going to announce who all the cultists are. And if you give them access to the cult QT, they're just going to post the cult QT to the thread.

A possibly interesting variation on night communication would be to have no QTs, BUT to allow the most senior surviving cultist to send a single one way message to the entire cult, sent in and relayed through the GM once the conversion vote resolves. If there's a Spy, they do get access to this message but do not convert.

I think that including a serial killer in a cult game would require a very large number of players. A vigilante would require slightly fewer people, but raises the question of what happens if the vigilante is converted - do they retain the ability to kill?