Campaign Theme: The Oblivion Gate

In the city of Equinox, the city founders have developed a way to deal with the excess of violent criminals in their midst and also their need for spies and agents for their ongoing agenda: using an ancient artifact called the Oblivion Gate, they have learned to erase the memories of those convicted of the most heinous of crimes. Those who agree to have their memories wiped in this manner agree to serve as agents of the state for a period of ten years, after which they will have their memories reinstated and be freed to resume their lives without further sanction.

Of course, that's what the powers-that-be tell those who awaken after having their memories erased. That doesn't necessarily mean it's true - after all, they've just had their memories erased, so who is to say they were truly guilty, or indeed the nature of their crimes.

For flavour reasons, PCs would not quite begin as a blank slate. Rather better is to have them each retain a single memory. (The in-world justification of this is that in order for the psyche to remain intact as the memory is wiped, they must fixate on a single thought. That singular memory, then, would be all that remains.)

So, then, when creating characters the players would not create a backstory for their PC. Instead, they should create a personality as normal but then decide on the one singular memory that remains. Additionally, the player should decide how the character feels about this lack of other memories: are they happy to wait out the ten years, do they want to find out what they did (or 'did'), or whatever?

For added fun, I'm also inclined to think that the players around the table should also each create a backstory, but not for their character. Instead, the DM should collect all the backstories, add one of his own, then shuffle the pack and have each player select one (but without looking). That then becomes the backstory for their PC, which they may or may not discover later.

The campaign would begin, of course, with the PCs being shipped out from Equinox to a rival city state, there to meet with their handler and begin their assignments. Where it goes from there... well, that's up to the players really.