Every month or so, the devs could add one or two little mini-quest to random NPCs. This would make the game an ever changing thing, maybe in small and subtle ways. so the person who has 'done it all," will know they really never will, because there are always new experiences waiting for them.
Here's how it works. The devs pick an NPC, which seems like a random NPC to us. They give the NPC a new mini-quest and choose a specific trigger, so not everyone would be eligible for the quest. This would keep people guessing which NPCs have the missions. I would not expect the mission to be found immediately, but over the months people would eventually stumble into them, and word would start getting out.
Reward would be random crystals, adventure techs of a tier that is appropriate for you level. Or maybe a random item from a long ago event that ended. This NPC's reward might be " Thank you for you assistance. Hey, I found this old mask, or shoulder pet in the warehouse, it's been discontinued so i cannot sell it, but you can have it."
For example, a quest might be triggered by:1) only a specific race of character of a certain level or class
2) only for characters who obtained a specific title, or titles. Like Defender of Trismus & Forest Skulk Slayer
3) or it mat only be available to characters below a certain level. Let's say your new character is only level 8 and you run her all the way to Bristugo, and an NPC there has a quest for only players below level 10
4) it could be linked to previous quests that were previously completed
5) available only at night or specific day of the week
6) or only for characters using a long sword, and the reward is a unique, tech kit that adds a plus to one hand slash, regardless of any techs already on the weapon
7) or the quest could be a treasure hunt type of event. Where you go to one place to find a note to the location of the next note, culminating in a nice little reward tailored to your adventure class, craft school and level.