"I want to mod a mafia game, what do I do?"
First of all you should probably have played a sufficient number of Mafia games to know what you're doing. If you try to mod without understanding how the game actually works then you're just being silly.
Let's just asusme you've done that. The next step is to pick a theme/gimmick for your game that you can design your setup around.
Let's say you're doing "Movie Mafia". Okay, that's the initial step but you need more than that to get started. In some cases the anti town factions will be obvious (E.g. Team Rocket in pokemon), but here you can have some more creative control. Want a Superhero mafia? Done. Morgan Freeman Movie Mafia? Done. The main thing when creating it is to make sure you get the numbers right.
A rough rule of thumb is that 10:3 is reasonably balanced for a single faction game, with a moderate level of town power. So in an 18 player game with 1 faction you can have around ~4 mafia. 3 very strong mafia, or 4 + a traitor with a lot of town power would also work, but sheer numbers might be two much. It's important to remember that 4 mafia are much harder to lynch and find than 3 -- it just needs one player in the mafia to play well for them to win. I'd use something like 10 town : 3 Mafia, 14 town: 4 Mafia, 17 town: 5 mafia and 20 town: 6 mafia, give or take as a rough guide for single faction games.
A lot depends on your town and mafia power, respectively. We'll get to that later on - but the above numbers are a reasonable place to start.
Hey, but I want two mafia factions? And a serial killer? And a cult!
Okay. Here's the thing, the more anti town factions you add the more luck based the game becomes. (Or at the very least, it requires different skills to play -- so bear that in mind).
In a game with an SK the game itself becomes more unpredictable - the serial killer shooting town every night basically means despite town playing and lynching well they can still lose, fairly easily. But if the SK shoots mafia then the game can end quickly despite the town sucking.
But, hey how is that any different than a vigilante? Well, it's different because the vigilante is part of the town - so him causing huge town damage/positives means he's playing poorly/well. But the SK is a wild card who just needs everyone dead, how generally shitty someone not aligned with you plays determing your chances of winning can be annoying.
Now, imagine it's two scum teams? Well it's amplified. Crosskills (when one scum faction shoots another) can swing the game dramatically (i.e. a town who had no chance of winning can suddenly become likely to win over 2 nightphases without lynching a single scum).
Okay, but I still want to use 2 factions
Okay, well whereas with a single faction game you probably want slightly under 25% (or 25% at max) here you want slightly more with a 2 faction game. In a 28 player game 2 scum factions of 4 seems ideal. (Yeah, that's just around 28%). I think any more than that makes it very hard for town to win. It's better to give them some sort of limited crosskill immunity (to at least one or two) than have 5 on each team and depend on crosskills to win)
Obviously then the more factions you add the weirder and harder to predict things become.
Okay, I've done that. Now I want a town cop, watcher, tracker, doctor, bodyguard, vigilante, masons, gunsmith, hider, commuter, beloved princess and scum to have a godfather, roleblocker, rolecop and framer? Is that good?
r u srs. Okay, the first thing is that more power roles aren't always better. Playing a PR
is fun, sure but it's almost better if it's not something that happens all the time. Don't be afraid to use vanilla townies or mafia goons. Surely you can substitute it for weak roles, but really a vanilla townie role shouldn't mean you don't want to play the game. If you need a role to play then perhaps you're missing the point? The game, should if possible, be the thing the game is focused on.
Too many info roles means the game becomes dull, with day phases shorter than night phases consistently.
If you ARE going to give everyone a role make sure of a few things
*That all the scum roles aren't all "obvious" scum roles to counter a cop. Don't be afraid to mix up roles, give scum things that look town, town things that look scum. I mean, okay. Giving scum 2 full vigilantes is taking the piss but giving scum an extra kill isn't the end of the world as long as you balance for it.
Similarly roles like rolecop can be useful to town, despite being traditionally scum roles.
The last thing you want is town being far behind and being able to confirm 4 or 5 or 6 people with a massclaim, it smacks of poorly thought out game design.
Okay, so I'm ready?
Uh, not quite. When you're creating your roles you need to figure out how they work with other roles?
So, what result does your rolecop get on your SK? Is that a desirable outcome?
How does your roleblocker work with another roleblocker and they target each other?
Does a gunsmith get a guilty on your serial killer? (Traditionally, no)
You should go through your roles and see if there are any problematic interactions. You can generally tell when something won't work very quickly. Either modify the roles or change it, or drop something -- but don't leave it in. Sure your rolecop probably won't target your SK - but it might? And then it's your fault, and you suck.
So, that role doesn't work as it doesn't match the flavour
Shut the fuck up, seriously. Flavour is fine. Flavour is good. Everyone likes an enjoyable game. You know what's not good? Making your game predictable and obvious. Okay, so what if X doesn't make sense as a rolecop? Not every role needs to 100% correlate with flavour. If it does mafia will have a very hard time justifying their claims. Yes, Die Hard can be a vigilante in movie mafia but not every role needs to be 100% "oh, that's so obvious". That just leads to trouble as a mod and for your scum factions.
Oh, and yes. Use strong safeclaims - or at least mix and match them so mafia aren't left with minor characters/having to forge ahead by themselves.
Things you probably shouldn't do but might anyway!
You don't want to give away game relevant information in your scene.
You don't want to tell someone when they're roleblocked. Just tell them they didn't obtain results. What if it was an ascetic? What if it was some other weird role? Why give your players more information on the roles in the game.
You don't want to interact/chat with the players in your game -- you might let something slip if you post in thread.
If someone calls you a bad mod, let it go and respond post game -- don't respond to them now. It confirms what they're saying has some basis in the current game.