Do you mean NPCs not standard in the adventure path, but introduced by DMs?
I foreshadowed the nimbusbow by adding a few relic hunters who were after Olman relics. The most notorious was a rogue/assassin called Heinrich Federweisser. He pretended to be a banker and I even had him finance part of Lavinia's expedition, so that he had an excuse to be on the Sea Wyvern. He was a member of a criminal organization specific to my campaign world. This organization is slowly splitting up into two factions, because the leader is not very efficient. Heinrich wanted to have a go at the leadership position, and hoped the nimbus bow would give him extra status and power.
I had a lot of fun playing him, since he talked very correctly and paused between every two or three words when saying something (except when he spoke Olman, which he did at a normal pace). This caused my players to cry out "hurry up!" in frustration many times and even avoiding conversations with him (except for one of them who spoke Olman) :-)
At first the contacts between the PCs and Heinrich were very beneficial for both parties, until they discovered his real purpose. They staged an extra stop in SWW at one of the Olman ruins along the coast. I developed a small ruin of a Tezcatlipoca temple which was guarded by a guardian jaguar and an olive slime. The ambush the PCs set up to get rid of Heinrich (involving this location and a fake Olman artefact to lure him to it) ended in a nice showdown between the PCs, the guardian jaguar, the olive slime and Heinrich. Heinrich of course did not survive (though he turned out to cling very tenaciously to life), but this was almost at the cost of the lives of two PCs. One barely survived a pounce from the jaguar and the other was already infected with olive slime when the cleric detected something amiss. It ended with the whole group jumping on the infected PC so that the cleric could heal him.
That was a very memorable evening.
Heinrich is not the only relic hunter. There will be some more during HTBM (they are already on the Isle of dread).
Sir Macario the Widower - a ancient and powerful death knight who was part of a Conquistador-type expedition to the Isle of Dread back in the age of the Olman empire and who assisted in setting off the original shadow pearl. However, an Olman princess he'd fallen in love with became a savage mutant, and he fell into a deep depression that's lasted the best part of five thousand years. He's completely disillusioned with Demogorgon, but still completely in his thrall. He recognises the PCs (and therefore didn't kill them the couple of times he's encountered them so far), much to their consternation, because they were present at the fall of the Olman Empire, due to a time-travel plotline that hasn't happened in-game yet...
Lady Irina Venkata - a noblewoman from Sasserine who wears black, never comes out in the daytime, has ebony hair and a pale complexion, speaks in a ridiculous Transylvanian accent, and sips thick red liquid out of crystal goblets. PCs have no idea whether she's really a vampire or not (funnily enough, they used a Contact Other Plane question to find out, and the dice indicated 'false answer').
Gilthoniel - weird elf passenger on the trip to the Isle of Dread. No useful combat skills or anything, just dreamy and enigmatic, but does sculpture in his spare time on board ship, and his subject matter is heavily demon-related. He seems completely blase, unconcerned and mildly curious about them, which has the PCs panicked that he might be a cultist of some sort.
I've also had an incredible amount of fun with Quenge. I've given him the whole Igor treatment - hunchback, lisp, the works. He calls everyone 'Marthster' and lives in fear of Avner putting him 'in the box' as punishment when he misbehaves...
How about a gang of diseased street urchins? Abandoned children who have found a home amid the filth and rubble yet have contracted ulcers, boils, and cysts. They surround a single person and beg for money or food, threatening to touch the victim if they are not rewarded. A few silver coins buy them off for now but they return days or even hours later. Such a disease may be fatal or just disfiguring.