Perhaps I didn't make myself clear in the OP (if so, sorry for the confusion). I'm not thinking about ways to get the PCs into a new setting. I'm trying to think about what they'd do when they get there.
They're too high level to get much fun out of killing rats in the sewers, so what do they do instead?
One of the fundamental questions that determines which tier (i.e. 1st-4th, 5th-10th, 11th-16th, 17th-20th) an adventure is written for is
Why? Why are the adventurers doing this adventure?
If the fate of an entire duchy rests on recovering the young duchess' wedding ring that was stolen by wererats while she was kept hostage (because the jealous duke believes it's a sign of infidelity), then delving into the sewers of Murkendrake might actually be a suitable 9th-10th level adventure.
Unlike a sewer dungeon at 1st level, the opposition is not rats but wererats, and there are many more complications:
- There is a thieves' guild seeking to start a war between the duke's land and the duchess' land. They were in league with the wererats until a recent falling out, and now they've sent an evil rival party into wererat territory to retrieve the ring.
- The wererats scout out their territory in giant rat form, and while in this form they behave as docile rats who are afraid of loud sounds and flame. If pursued, they flee...back into a cadre of hiding wererats with waiting knives.
- There are devious poison gas and crushing block traps in the sewers that require diving underwater to disarm/deactivate.
- One of the recently afflicted wererats is the duchess' troubadour who was secretly her paramour before she was married. He serves as the wererats' liaison to an underground beggar community, attempting to fight his evil nature. Determining what to do with him could be a moral dilemma.
- The duke's elite guard have been charged with smoking out the wererats, guarding all major sewer entrances and after so much time will start fanning smoke into the sewers. The guard don't realize the duke has sent them because...
- The leader of the wererats has blackmail material on the duke – a ledger proving that the duke had a personal hand in destabilizing the economy of the duchess' land, which is what led to her capitulating to marriage.
- There is a secret submerged passage leading from the sewers into the Murdendrake prison, where the duchess' brother is being held as captive of the duke, who has sworn to chop off his head if their wedding ring is not presented by tomorrow's full moon.
The setting is familiar, but there's time pressure, more dangerous hazards, rival parties & factions.
The monsters are familiar, but they're present in larger numbers, more cunning, and with more plot weight.
The premise – a dungeon delve to retrieve a McGuffin – is familiar, but the scope, the
why, is very much a political quagmire with multiple potential solutions. The stability of the realm is in question.