This is a mystery/role-playing game. It takes place in a little town, which has been quarantined after the discovery of a changeling infestation.
Players would be divided into two teams: A Citizen team, and a Changeling team. The Changelings would need to be substantially smaller than the Citizens: say about one-fifth as large. One player will be the Storyteller: he assigns roles and teams, and keeps track of characters' jobs, and counts votes. The goal for the Citizen team is to get all the Changelings locked up in town hall. The goal of the Changelings is to whittle the players down to the same number as the Changelings.
Each round would take place over a number of Real-World days (2 days for "daytime" and 1 for "nighttime" would probably be optimal). During each Day Round, players are free to Roleplay, and discuss who they ought to vote out in the comments. The Changelings are free to discuss who they ought to abduct next in Personal Messages. By Midnight on the west side of the International Date Line, each player (both Changeling and Citizen) should have cast one vote as to which player should be tied up and locked in city hall. The scene involved should be suitably dramatic. During each Night Round, the Changelings must tell the storyteller who they wish to abduct. Other players with special roles will also do their jobs.
Some other possible special roles would be:
The Detective, who can look at one other player during the Night Round to see if they have a role, and if they're a Changeling.
The Sympathizer, who is not herself a Changeling, but only wins if the Changelings do. If the Detective looks at the sympathizer, she'll look like a regular pony.
There are probably a lot of other roles we could have as well, but I think this text block is big enough.
Please discuss below!