Well...I guess I have an idea:
How about a colliseum?
I mean, a colliseum to test how strong you are, fight against friends, summon battles, fight against enemies, ect.
Don't know if it has been mentioned before, so...
(By the way, this is skipper from purezc...and darkmog from ZFGC-X or whatever.)
Oh, and another good idea: Quests. Like, every now and then, you can make a new "mission" for players. Like, lets say there is a place called... hm... Rufia Mountain. Like, one week, have some mission like "Journey into the dephs of the mountain to retrieve the sacred sword" or something.(That was a poor example of a mission...
)
Anyway, I still think the colliseum idea is the best. ;D