The Agaria is controlled by a gang, very much like a collegia of ancient Rome. The Tyran of the Agaria (each area of Illusidum city has a Tyran) has a problem. Someone has come into the Agaria and is mugging citizens at night. There have been three robberies already, all three victims killed. The quest is to locate and either capture or kill the thief. The Agarian Tyran offer a reward.

Ides stage: suggestions.

perhaps you have to find clues by asking different people about it and they then lead you onto the whereabouts of the murderer.

That would tie in some more NPCs and help fill the Agaria. Yes, good one.

It could be the mugger has a hidden route into the area, also useful for a quick exit if anyone gives pursuit. There could even be a chamber in this that the mugger uses to put on a disguise, so they appear as a normal person afterwards.

The clues could initially be misleading, as the mugger uses different disguises, but they could all point to the same route. Maybe even there could be more clues to the mugger's home turf; is he part of a rival gang or freelance? Can the clothes be traced to merchants who all describe the same buyer? If a gang member, the quest could be to infiltrate their area to do the hit, so as to send a strong message to the gang to not repeat the behaviour.