If i go back to the old Sro times, i remember some feature called "Pandora Box", It was a scroll u could open and from there on different strong mobs spawned with increasing level(1-80).
That was a realy cool thing and i did not see it on any other servers, so maybe u can add it as a reward from Ctf tickets for example
or do a Pandora box event
(so nobody can abuse this scroll to kill other players by spawning mobs near them)
The event can look like following:
1) the players who want to participate on the Pandora event need to form a party
2) for each participating pt the gm spawns 1 Pandora box only for the pt
3)if the pt can deal with all mobs, they get a reward ( but dont make it to easy)
You can also adjust the mob range to the max level of the pt member, so everyone can participate even low levels
