jolly pumice
I'm not talking about RP quests. In D&D Online (And i am sure there is other games) there is a feature that lets you create quests & stories for other players to experience. With the game currently this could flesh out the world a bit more, keep people interested & doing stuff, however it is something that'd need a lot of work.
XP Calculations
Rather than just giving a player X amount of exp, it'd need to be based on a value system where X npc is worth Y exp & multiply it. Giving the quest more grounding. If you make it "give what ever xp" then people are obviously just gonna give 1 billion xp for killing a larva. No fun.
NPC's
The target NPC's you can work with a dropdown menu, & you can also create "Story NPC's" Which spawn a temp npc for the player that can build some kind of narrative / "Chapter" to it. They pick where the npc will spawn, what it'll say, how many of said Enemy npc's they need to kill, etc.
Community
These quests as i said will help make the game a bit larger, It'll help the team find quest makers who have good ideas for quests & make their custom quests "Canon", but it just gives you something a tad more interesting to do. Have the players quests be able to be voted on so popular & unpopular quests can be spotted, Rank their difficulty, lvl range, etc etc.
The most difficult part about this would probably be both the creating & spawning of "Custom narrative NPC's" & spawning them about the world, but the framework of everything else should already be in the game, Just code for spawning lists & creating a custom quest, etc just needs to be added & it should work fine in theory.