I had an idea that might encourage players to rotate through some of the old maps so they don't simply die off except for the occasional new player. If each may had some significant once a week reward or chest that could be opened for an event and those events were staggered in the weekly refresh then it might encourage players to rotate through the maps over the course of the week. So Ascalon could be promoted on Mondays, Blood Legion on Tuesdays, etc. etc. Then instead of having continuing Boss trains you could promote short runs through each map on various days. If you put it on a 6 day rather than 7 day cycle it would also change the date of preference for what day of the week the events rotate.