I think going the PM route would start to make things too complicated again. Simple simple simple! The murder mystery idea is great, but I don't think it's necessarily simple.
I think something like I suggested initially would work well, no? Or perhaps even take it up a notch and, for example, have a group of friends go somewhere such as an amusement park, a bar, etc. with (or without) a specific goal in mind. This kind or RP could be done either as our forum personalities or we could make a simple character creation/plot discussion thread and prepare accordingly. Should we go that route I think character descriptions should only be about a paragraph in length (i.e. very brief). Age, sex, and a quick summary of their personality.
That's what I'm thinking, anyway.