This would make the game really more interesting and make towns even more usefull.
You know that on town, there are npcs who does nothing else than just say something useless, or related to the game lore.
So I were thinking here, those npcs could unlock quests depending on how you progress on the main quest, by finishing the main quest chapters (this is the easiest way of making the player know that may have unlocked a new quest).
For example, after clearing the Tenet Chapel chapter, a man in the town could offer you a quest to do something in the chapel, a woman could ask you to bring flowers to her husband’s grave in the tenet garden… And even more, and offer exp cards as reward, and if possible, offer something that is usefull for the player, or even unlock something new.
Other quest could be of unlocking something on stores, for example, a quest that appears when talking to the Item Shop woman after you find the Goddess, which when completting the arc, allows you to buy level 3 health and mana potions.
Another kind of quest could happen once you enter the town, like a town npc is talking to the other, and your character interrupt their chats, asks for information, and after the information is given, you choose if you will or not help the npc, if you choose to help, a new quest will pop up.
The most interesting part of pop up quests, is that repeatable quests that appear randomly (after a certain cooldown, obviously, since we are on a mmo, let’s not overload the server with peoples entering and leaving the town) so you can farm exp, silvers and even exp cards.