What level is the party? This will likely dictate the type of quests that the guild will ask of the group/trickster. What is the guild and/or the player's primary alignment? While I enjoy a good old fashioon, go smack some goblin heads around, but often as a DM, I like to think of things a little less straight combat forward as this.
Some options
1) This will change in significance based upon the alignment and level questions above, but perhaps the guild's immediate need is not so much warlike. Perhaps they are looking to expand some of their storehouses, and require somebody to ..."convince" some local poor inhabitants to vacate a row of shanty shacks in the slums.
This may force internal conflict on a good PC to essentially evict poor people from their homes. It also allows for them to think diversely and use methods out of basic combat. "do we use bluff to convince them that we are pest control sent by the city to perform extermination?" "Do we Intimidate or bribe the families to leave?" etc.
Then you could sprinkle in a combat with the idea that goblin spies were watching them the whole time, waiting to attack and set up their own camp in the vacant lot, or that the guild in an act of betrayal just needed somebody to do the dirty work, so they could sweep in and save face by "handling" these insensitive people.
2) Have them perform an old fashion robbery. Stealth mission into the ritzy neighborhoods of magnimar. Break into an aristocrat's mansion or a long dead noble's mausoleum, trying to take care of any guards and recovery treasure, or simply to map it out for other thieves.
3) Have him meet some pompous a+~++@& thief from the guild who is demeaning and arrogant. Just waiting to be challenged in the skills of theft and deception. A competitive race to pilfer as much coins as possible, only for the guy to break the rules and try to murder and steal from the trickster.
In my experience most players build a character for combat, but always enjoy role-playing and story more. Put at least one combat in the side plot, but try and focus of some of the rogue's skills, disable device, bluff, diplomacy, interacting with both the rich and the poor, as well as trying to steady their moral compass.
good luck
Season four of Pathfinder Society was set in Magnimar. There was an early subplot involving goblin tribes and some underwolrd groups. I'd suggest checking out PFS 4-01 to see how that is setup. It even involved a sewer excursion, so you can probably mine it for ideas.