Bump because this topic is interesting.
I find that games with 2 mafias immediately screw one side over because of mass kills. SKiller, seeing as he has no teammates to take into account, is more balanced for a game, but I have never seen a SKiller win and think that the role is doomed to failure.
I feel that 3rd parties are generally shafted because the only ones that can win are the ones that piss people off. Those being Jester (objective to get lynched) and Assassin (Objective to get specific someone/group of someones lynched). Other than that, the third party win objective tends to be to survive. What other 3rd party win objectives have you guys seen?
My favorite set-ups are those that include a heavily powered mafia and a small group of town power roles with a large number of "apprentice roles" that inherit powers upon the death of a town power role. I've played two games like that and hosted one. All three were extremely interesting (one was broken by a bunch of alignment switching roles though).
It's also been my experience that the game is generally stacked against scum and that town wins waaaaay more often than scum. The margin of error is so much larger for town than for scum.
I don't know what you mean by "mass kills," but usually games with 2 mafia factions don't tip immediately in one favor or the other like you say you find they do. If you look for examples, you might prove yourself wrong!
3rd party roles do just fine as well. In the games I've read here at SWF, either the SK won the game, got **** close to winning the game, or was Junglefever. When you ask about 3rd party objectives besides either killing or surviving, you have to remember that mafia's objective is to kill and both mafia and town's objective is to survive, so you really shouldn't stray too far from the game with an independent role. Lynchers can easily be broken and not fun.
I also don't know what you mean by the game being stacked against scum. Maybe if you think that, just take a peek at the name of the game designer, and look at the other games they've designed. But if you simply go through the archives, I think you'll find that scum wins just as much as town... the fact that the margin of error is so much larger for the town makes sense, don't you think?
The fact that mafia's goal is to cause x number of mislynches in the game would mean that x should be greater than y, the number of correct lynches town needs to win, especially when town outnumbers scum!