Since Think was interested, I'm going to post one of my gametypes from my list (and the process I used to create it). It's not finished, but it's somewhat along, and might even be a decent game. This is pretty recent, so I still remember my thought processes for the design stage.
I started by thinking about a game where everyone is serial killers, and how not-fun that would be since there's no real strategy to it. Everyone kills everyone... but there might be a way to get it to work. Bang! has town and scum, but everyone is still a killer, how do they pull it off? Bang! has a lot of similarities with Mafia (look it up if you don't know it), but it has proximity rules. That's a lot harder to do in a mafia game, but... maybe instead of having players next to each other, we could have them in some sort of rooms, and they could only use actions on players in the same room.
And players in the same room - is there anything else we could do to make it more interesting? What if they were allowed to talk to each other - even during the night... Hmm, this could work, especially if they can move around a lot. Okay, what other considerations are there?
Implentation: How would this work? Well, there still needs to be some sort of main thread activity, I would think, and the rooms don't support a day game, so let's just get rid of them for the day game. The players will spend the days in some sort of central hall/building/something, and then break off into rooms at night. Then... yeah, we can create quick chats for each room, and switching to night involves getting sent the link to that chat. Players must post with their own name, and they cannot post in rooms they aren't in.
Actually, to prevent being able to see what happened in a room when a players not in it, that's a quick chat per room per night. Players cannot post in rooms from previous nights.
The first post should be the host saying who is in the room - who can players see that are valid targets for their night action.
Does this make their night action visible to other players? Maybe... this is going to need balancing later, so lets not restrict our choices, and say there might be both visible and non-visible night actions.
So lets think of some abilities - I like power roles, and without some interesting power roles the rooms are just weird night-chats, which isn't enough. So how about...
Follower: Day action - you follow target player to whichever room they choose to visit during the night.
Shadow: Passive. You are not included in the player list for a room. You must still make posts under your own name if you choose to make posts.
Brutal Killer: Night Action - Kill target player. This is visible to other players in the room.
Poisoner: Night Action - Kill target player. This is NOT visible to other players in the room.
Delayed Poisoner: Night Action - Target player is killed at the end of day the next night. (This is great for misdirection, since people will assume they were killed by someone in the same room)
Restrain - Visible roleblock.
Paralyzer - Invisible roleblock.
Lights Out - Inisible roleblock against all players in same room.
Exploder - Kill self and everyone in same room.
Thief - Steal something from someone in same room (assuming items serve a purpose)
Etc. and so on.
So to make things interesting, we've really got two major considerations - Visible versus invisible abilities, and target multiple same room target abilities. The first isn't just whether the user is seen, but when and where the effect is seen. Most standard abilities obviously work as well, but only target same room folks.
We might also consider giving the rooms themselves some sort of abilities. Or something to differentiate between them. Somewhere between 3 and 5 rooms, depending on player size. Or perhaps have player abilities tied to specific rooms. Most of these considerations will probably require us to have a better idea of the flavour to really make them work though. Are we on a spaceship, and the Engineer needs to spend three nights working uninterrupted in the engine room to fix the ship? Hmmm...
Okay, that's a decent base - lets move on. We'll decide on game size and specific balance issues later. First, gotta get some ideas for some thematics.
Classic Clue Style - Big mansion. A murder. Find the culprit, but with a bit of... ongoing violence. Library, Kitchen, Greenhouse, Garage/Stable, Cellar, Attic. Make it a haunted house for rooms with more... personality options.
Spaceship - Central hub, various rooms. Greenhouse, dormitory, entertainment room holodeck, exercise room, engineering. Lots of fun gimmicks we can stick in these rooms, too, since those would be Space Plants, Space Beds, Space Holograms, and Space... Treadmills?
Museum - Lots of fun potential rooms, especially if players are all exhibits from a certain room that come to life and wander the museum at night. Even a group of nights guards might be interesting - perhaps some ancient artefact was triggered, trapping them all inside and they are exploring the museum trying to figure out what happened. Lots of potential fun rooms - too many to name, really.
Something Monstrous - Perhaps the rooms aren't so specific. The players could be some sort of demons/boogeymen/monsters, and the rooms are places they can be summoned to, or places on earth they can visit, or beds they can crawl out from under before returning to hell/the court/the trash pile each morning. I'm suddenly thinking of the potential for an Aaah! Real Monsters game.
Something More Surreal - The rooms could be planes that ghosts travel between, or some part of a persons mind and all the players are their multiple personalities. There's actually a ton of options to build off this.
And that's the state the game was in when I stopped thinking about it. If it's not clear, components of many of the possible themes are drawn from movies and other media, usually with some sort of twist. For flavour, I do this pretty commonly. My "The Thing from Outside" game was almost a direct rip of the Doctor Who episode "The Crystal Planet". My Pokemon Box game had obvious inspiration. My Starsiege games were pulled from the stories of Trojan Horse units you get over the Omniweb during the Starsiege game, and my recent Politibastard was based on the circus known as the Republican Primaries. My "Repercussions of Evil" game was particularly fun.
And there are plenty ideas out there ripe for the stealing, yet! That doesn't mean you can't come up with something original, of course, but even that could usually be improved by a common cultural element players can recognise (in my opinion).