[Spinball]

Ok so a conversation is displayed like a thread, yes, so you can follow it much more easy than PMs. But you can also invite people into the conversation, so they can read it from the start. And then remove them from it if you want. You can also specify the permissions whether people invited into the conversation are then able to invite other people.
Beyond this, there will be the ability to report conversation messages to the moderators to be handled by the reported message system.