The basic premise is this: there are two factions (party protectors & party crashers; names up to a given DM. One could be "The Royal Guard," and the other "The Thieves' Guild," but protectors/crashers is general, and gives a better idea of their role(s)) and the game revolves around some sort of party (I'm thinking masquerate mainly because the masks would help conceal idenities and make for a more interesting game). This means that it would be more primarily leaned toward roleplaying that rollplaying, and of rollplaying, more leaned toward charisma/intelligence/wisdom skills than combat prowess.
Two to three players would take on the role of the party crashers. These people have come to the party for a few reasons, but the general idea is to cause mayhem. And I don't mean "start randomly killing people," I mean, "We've been given these tasks to complete... "
Their mission, if they choose to accept it, is to fulfill these objectives: they can do it quietly, quickly, brazenly, meticulously, or whatever you want to use to describe completing the mission.
In addition, maybe require each PC to have a personal character goal. To give them a greater IC stake in things, greater reason for wanting to accept, and so on.
For example:
Party Crashers: Thieves Guild
- 1: To steal a priceless magical necklace from the host of the party.
- 2: Retrieve at least 5000gp worth of goods to be fenced.
- 3: Setup (person x) in some way or otherwise implicate them in a plot (murder, theft, treason).
- 4: Embarass the host publicly in some way.
Party Crashers: Assassin Guild
Perhaps they have a list of 4 or 5 targets who are attending the party that they must kill; some having specific requirements.
- 1: Kill (noble x), making it look like an accident.
- 2: Kill (noble y), however you wish.
Party Crashers: Personal Goals
In addition to the objectives given them to whoever hired them, personal goals could include...
- 1: Retrieve a heirloom that one of the nobles stole/cheated them out of.
- 2: (Noble X) will be at the party, a desire for revenge for said person having PC's family killed. Naturally, how they get that revenge is up to the player - who knows, they could work it into their objectives...
On the other hand, Party Protectors are there for similar, but opposite reasons. They could have been hired by the host to make sure everything goes smoothly, or hired as bodyguards for specific nobles (that are targeted by, say, the assassin's guild). In addition, whose to say they couldn't have thier own personal goals? And their goals may not exactly be up to their role, but then that's some of the fun. Can they (using above examples) get revenge while protecting their ward? Steal that heirloom back (or find and persaude the noble to return it) while making sure the magic necklace isn't stolen?
Essentially... It's a social/RP Player-vs.-player game instead of some player-vs-player arena combat game.
The DM would be responsible for keeping the party moving, making sure nobody tries to cheat in some way (like metagaming, or somehow being able to cast Dominate Person at-will), making up scheduals for given target NPCs (the rest don't mean as much, but they'd need to keep in mind to give opportunities where these NPCs are vulnable, and times when they are a little more guarded to give both groups chances for personal as well as group-oriented objectives).
As for other considerations - Now, this may not be a long-term campaign in itself... but perhaps it could be turned into one. Maybe it is The City Guard vs. The Assassin's Guild. Each 'adventure' provides similiar opposition. A few days later, the Party Protectors could need to escort a prisoner across the city... the party crashers need to make sure this prisoner doesn't make it (or free him). The party could also last several days, to a week, which would provide plenty of opportunity and the goals of a given day could change (perhaps NPC x isn't there on day 3, but NPC y will ONLY be there on day 3 so he HAS to die then!)
I'd say, for fairly clear reasons, neither side would be able to see the other side's character sheets (but they could see the sheets of their allies).
At the very least, I think the party thing could provide for an interesting one-shot game.
And I think that's all she wrote on this one for now.
