We actually had something like this for a short time back in the day. I think they were called "Event Masters" but don't quote me on that. They were separate from GMs and were there mainly to lead role-plays. I don't know exactly what happened to that idea, but I suspect that when you have a limited supply of people who are both willing to help out and qualified to do so, it doesn't really make sense to restrict them to a specific sub-role. I don't know how it works now, but when i was on the GM team we were selected primarily for RP interest and (perceived) skills, which actually makes a ton of sense; This way, the lowest level GM's can run RPs if nothing else, and those who gain higher ranks are given moderation "privileges." You're not arbitrarily cutting off your supply of those who can lead RP's, and only those who are the most trusted become, additionally, administrators.