This could become a module in the future, but we still have a lot of work until we reach that point. Also, this certainly must be optional because not all apps will find this feature useful. We had a rating system in the old Nabble, but we removed it from Nabble2 because it affects user ego. So we won't work again on any system that can rate someone down. All rating systems from now on should only rate people up. But, as I said, we can discuss this when the module infrastructure is ready.