This is a very open "design fun" challenge, where the only goal is to add as many useful/necessary NPCs to a town as possible. Basically when a player comes into town they should have "errands" and similar that they need to do before they leave it.
The goal is to obviously add as many useful NPCs as possible. If you need to add an "otherwise bad mechanic" to do so you're welcome to.
The gate guards. You are given a short branching QA type interview. Based on what you are doing and based how you answered you could end up with an entry tax, or further mandatory quests.
Players can't loot in PvE, instead they fill out "ledgers" automatically (that thematically have location of corpses, the path they cleared, and similar). That are then sold to NPCs for the loot in town. For instance if you wanted gold you would take it to the bounty office, if you wanted alchemical ingredients you'd bring it to a botanist. (could easily add 5 to a dozen NPCs to a town).
Coin Changer. Each town has a different currency, so if you want to trade without a cost penalty with merchants you need to convert.
Healing is hard, especially that last 5%-10% of your missing health. requiring a combination of NPC medical professionals, and places to stay like an inn.
Adventurers Express. If you know your endpoint you could pick up some extra coinage by picking up a package here for delivery to your destination.
Practice doesn't make perfect, practice makes permanent.
"At one point technology meant making tech that could get to the moon, now it means making tech that could get you a taxi."