I'm totally all for a mafia! (I didn't last long at ALL last time XD). It was a lot of fun to follow, and I looked forward to updates to see who the mafia were. I already have a character idea I could use ;) Whereas I prefer full-on RP for story immersion, RP-optional would allow those who don't want to RP to partake, too. And the more the merrier!
EDIT - To avoid over-complicating things, can I suggest a rule for non-RP comments? Maybe have them written in parenthesis/brackets? That way, those following the RP will be able to do so without getting confused.