Oh, that's not a problem.
That means your PC's are probably the biggest guys in the whole valley: they no longer need to go looking for adventures. People who need help will come to them.
Dragon raid? Kobolds getting uppity? Random magic tower spontaneously appears? Call your party, because they're heroes! You don't have to take the party too far out of the city, but you might let them get out to the suburbs.
I've not played the module you described, so I don't know any campaign backstory. Thoughts I have, however...
1. River pirates! The scattered bands of thugs up and down the river are rarely more than an annoyance to any competent bargeman: three guys row towards a barge in a canoe, lob a grappling hook at the rail, suck some crossbow bolts and a vial of alchemists fire, and sink.
When the Ebon Cabal begins selling them magical support and healing, they start to be a real threat to shipping.
Adventurers with a flair for investigation might find stolen goods for sale in the marketplace, then trace the goods back to the cell of the Ebon Cabal responsible for monetizing their share of the loot.
Adventurers without a flair for investigation might rig a barge up as a Q-ship, subdue some pirates, and interrogate them, following the trail the other way.
2. A protection racket for the whole town. The Ebon Cabal is threatening to cut off all the river trade using unspecified dark magic. (I like planar binding evil water elementals, myself.) The party can participate in the negotations, track down the ransom payment and try to take out the cabal or, if less thoughtful, can just kill the bound monsters.
3, Taxes! The secular government wishes to tax religious establishments. As the only legitimate and openly run religious establishments in the city are the Luminous Order, this is a de facto tax on them. The Luminous Order suggests confiscating the assets of the evil Ebon Cabal instead, and offers help in discovering and recovering those assets.
4. Sewers! The PC's have eliminated all nearby threats to the city: as a point of light, it's shining brighter than ever. More people are coming to settle in the safety of Slaughtergarde (which might want to rename itself to something a little more cheerful.)
Now the town needs a sewer. Fortunately, divinations indicate a network of caves deep under the city. The city is willing to sink a shaft to the caves, but brave adventurers are needed to go make sure the caves are empty of monsters. (Because any monsters in the caves will be unhappy once everyone starts emptying their chamber pots into their lairs.)
How are those?
