I'm creating a Murder Mystery RP, but as this is the Interest Check, I'm going to keep this bit fairly simple. You get two characters, one of whom has received this invitation by whatever means suits you (mail, found in a gutter, stole from someone, etc). This is set in the modern time, with Riverside being a small town (village) along the Ohio River northwest of Pittsburgh, Pennsylvania.
The interesting bit, at least for me, is that after character creation (remember, you get 2), I will do 2 dice rolls. One will determine which character will be murdered during the story, and which character will be the murder. It is possible that a writer can end up having both the Murderer and Murdered in their characters (or even a single character if you take into account suicide). At that point, I will contact the writer of the Murderer to flesh out the details, motive, etc.
Then the story begins, with the characters surviving this situation, attempting to find the murderer (or elude the others), etc. Once the initial murder has occurred, the entire story is completely fluid from that point on, meaning everyone could be murdered, or the murderer could be discovered two minutes later. It all hinges on the skill of the writers and their characters :D