Here's some ideas that I used for my own group of 5 level 9 PCs based out of Sigil, that soon expanded to 6 level 9 PCs.
Travel to a prime world (Toril) to find something known as the Codex of Long Shadows and Last Breaths buried in the Great Barrow in far north east Faerun. Dead of winter, open plain, angry ghosts, and bound fiends. Very fun when they did it, and it's the next plot arc being written up for my 2nd storyhour.
Travel to Acheron to speak with Teslor, offering him some of his own memories or perhaps a momentary respite from the oversight of Kai Lung, in exchange for details regarding Choirosis the Great Wyrm rust dragon in nominal command of the Sodkiller fortress on the cube(s) of Marsallen. Eventually make their way onto Marsallen, find a way into the archives of the Mercykillers still contained there, and find a specific box dating back about 200 years with records on an adandoned and buried program run by the Mercykillers, Fraternity of Order, and Harmonium known as Object 105.
Travel to a specific prime world to assassinate a low ranking countess in an otherwise obscure prime kingdom, recently grown fat on supplying weapons and soldiers to two neighboring, warring kingdoms. She herself isn't important, but she's a devoted worshiper of Bel and the PCs employers want to send Bel a message. Bel, or his underlings in his court, will not be pleased, and the PCs will have to pay for the infraction. Up to you if the PCs know she was aligned with Bel in the first place. Mine didn't till they chased her Imp familiar down a flight of stairs from her bedroom where it had fled to a hidden room with a shrine to Bel and some sort of sending stone device.
