Hmm, so let's see.
Three 'permanent' games:
Paranormal (it's what brought me over here, and I'm probably not the first to think of this as the trademark 'bay12game' mafia)
Beginner's Mafia
Plain old mafia
1 'testing' mafia (a mafia that's currently in beta. I honestly would put Religious as one of those since it still needs a good deal of tweaks. It also makes sure we don't get 'locked' on the same mafias over and over)
After that we should have a system as to which games should come up next based on demand. If 20 people are rioting in the streets for another Toon mafia, then we shouldn't stop them, but 5 people going 'alright' isn't enough for Santa to even get a thread. We could have a voting system in a thread with suggestions for mafias and a mafia would have to get a certain number of votes to start up. A poll would work best since, otherwise, it would be a MESS of posts with "IN!" filling the thread quickly. I know Scummafia does that but they have subforums to make it easier.
Idea:
Thread is opened and, by Monday, Three people suggest their mafias and requested number of players: Bay 12-requiring 15 people, Wizard Dual-wanting 12, and Alexhan's Choice-wanting 12. Poll lasts until Friday. On Friday, the mafias that get as many votes as requested players get the Green Light. On Monday, the cycle repeats.
That allows for some control in what games get started. You get some partial control over how many games people join by controlling how many votes people get (thus even if you want to play in all three you can only vote for one and would have to hope 12 OTHER people want to play in the others). I'd set it, personally as: 1 vote per person, you can change the vote, 5 day time limit, anyone can see the results, and no more than 3 listed per session.
@ Mr. Person
I'd put that 'Alexhan's' mafia as a special mafia, since we've all grown used to settings like No Hammer. I imagine 'smalltown' is the same as what I call 'Plain Old Mafia', set like Begginner but with open enrollment. To be HONEST, I wouldn't mind if someone else ran it so I don't end up with a mass of mafias to take care of. If no one else does run it, I will, but I'm content running Beginner and, perhaps, some special mafia.