ok, so i been thinking and i got 2 ideas that you might like, now 1 of the 2 will be a Sh*tload of work if you implement it, BUT, believe me, you will have the best private server EVAR.

Idea nr 1 (Seems Simple enough)
Titles.
Make an NPC that gives various quests to defeat someone/somthing or whatever, upon completition the player will receive a title, if he puts this on 4th slot - he will have a title added to his name.
EG:
Objective: Kill Squidy mini boss
Reward: Squid Killer

How it will look:
Title: Squid Killer
Name: BadKarma
Clan: Eclipse

-------------------------------------------------------------
Idea Nr 2 ( The Hard one, and i mean - REALLY HARD)
4th Rebirth, yes you heard that correctly - 4th rebirth.
This however no longer will be a traditional rebirth - oh no.
as a matter of fact i think this rebirth shouldnt be available in IM.
Heres what i imagine at 4th rebirht: "Choose your side, Heaven or Hell". No longer mixed wings, when you initiate 4th rebirth you get to choose what side you will join - Heaven or Hell. The quests will differ, if you complete heavens rebirth you will have all angel wings and if you have completed hell's quest you will have all demon wings. Also, i think it would be a nice touch if the abilities given as reward after completing the quest also differed, EG Heaven would be focused on healing and buffs while hell would be all about attack and debuffs.
Also, upon completition your name colour would change from white to either red(Hell) or yellow (Heaven) to show you have completed it.
The quest itself should be hard in objectives but i can imagine that you could make the objectives short and easy to make for yourself. And i want to stress that this shouldnt be available in IM and that all items required to complete it cannot be traded, that way you will have only dedicated players completing the quest.
I am sure that if you were to implement this you could end up with more players than ToP. IMO if you are capable of making maps, you should be capable of doing this. maybe objective should be like: Complete Maze 1,2,3,4,5,6 and retrieve boss item drops or summat like that.
This would be EPIC.
--------------------------------------------------------
And the last idea, im not sure if this is possible, but then again - im not sure whther 4th rebirth is possible too...

3rd Class selection.
This too would make DSO that much more unique, a third class would be great, eg: Crus can advance to paladin or uhm...dervish (lol wut o.O). one would be concentrated on speed and dodging while the other would be about attack and defence. what i ould imagine is:

Swords man->Crusader->Paladin (More Attack, Defence)
->Crusader->Dervish (More Dodge, ASPD)
->Champion->Brute (More attack, 2 swords instead of 1 and high def)
->Champion->Golem(IMMENSE DEFENCE, but only 1 two handed sword and less attack than brute)
Explorer->Voyager->Pirate (Sea voy, more based on sea, sea skil)
->Voyager->Land Rot(Land Voy, more def and SPR and land skills, more balanced with other classes)
Herbalist-> Cleric->Heal master (Massive heals, few buffs)
->Cleric->Buff Master (Focused on buffs only, soem heals)
->Seal Master->Seal Expert(Focused on debuffs and sealing)
->Seal Master->Dark Mage (Focused on powerful spells)

Hunter->Sharpshooter->Assasin (Bow, good damage great speed, maybe stealth?)
->Sharpshooter->Gun Nut (Guns - main stat is accuracy)


Why do a 3rd class advancement? Well here what i suggest, aside of only getting a buff on current is to up the max one can add to his stats, so instead of 120 it owuld become 140 for eg ACC for a "Gun nut". also, i would suggest to not put this in IM or job changer, but have an actual hard quest for it.