Ok...Here is your dose on inspiration.
The idea is to have 3 mafias battling it out. In a Smash Bros themed game the mafias could be different gaming worlds battling to see which game universe is the best.
The game would end not when all other mafia members and townie members are below a certain number, but will end immediately after there is only one mafia left. This is because there will be significantly less townies...and the idea is the mafias are battling each other and not townies. That is not to say voting will not be important though.
In this game the mafia who has night action is dependent on which mafia was hit in the last night action.
So if Mafia A killed a member of Mafia B in a night action, then Mafia B will get the next action. If a townie is killed then a random townie will be given the power of night action for revenge. But they must hit another mafia or be killed themselves. That means there is a vigilante role that can not be eliminated from the game until all townies are killed. So it is in the best interest to hit townies.
If no action to kill is taken then the host can randomly pick a mafia to townie to act.
So this creates an interesting flow of give and take to the game...and seeking revenge for actions.
Finally having an investigator is a give or take. For townies it could be a helpful power, but not necessary. With so many mafias it isn't going to be too hard to spot villains. However, it could help in trying to work together...so I guess I would leave it in.
Depending on how many players there are the mafia teams need to be at least 3 members each.
So a minimum game would be: 3 mafia 3 mafia 3 mafia 5 townies 14 people. If you got to 21 people you could plus the mafias 4, 4, 4, 8 townies