That sounds like a blast!
I've been running a steampunkish campaign for a few years now (my first campaign). Fantastical races, fading magic, psychic imperial agents, and so on as a background for a Firefly-style airship campaign. Fairly early on, I realized that the templates (classes) my players had chosen were supposed to be advanced by seeking out reclusive masters to learn from, which doesn't mesh well with a nomadic campaign premise. So, I hatched a plan (any crew of the Salutation, stop reading here!):
They were hired for a B&E job to recover some property: a gemstone in a lead-lined box, which they were warned extensively not to open. They rescued a manipulative hostage, beat the bad guy, and when searching for the goods, knocked the box down and it popped open! This led to a quick scene switch where they were bombarded with memories, sensations, and impressions from a myriad of minds. Each PC was gifted with memories of secret techniques and knowledge based on where they wanted to take their characters (along with access to a Psychic template), and were invited to journey north to join the Communion fully - this was just a taster!
One of the PCs managed to close the box back up, they turned it in for the reward, and ended up with a few passengers who had also been caught up in Communion's invitation. The PCs were a bit creeped out and not really interested in joining up; but some of the passengers asked for passage in exchange for work, so the crew has been working their way generally north while they engage in other adventures.
As they journeyed north, I've been dropping mentions of "Brass Walkers" threatening towns and cities in that direction - mostly rumors and broadsheets that they don't trust (everything official has a pro-Imperial bent), but they also worked with a dispossessed noble who lost his holdings to them. What generally happens is people start spotting men and women with strange metallic prosthetics on the outskirts of a town, people get spooked and prepare to hold them off, fight valiantly, and then some of their neighbors start unbarring doors, sabotaging equipment, and otherwise turning coat. Sometimes, survivors will recognize some of the Walkers as old friends and loved ones from nearby towns that have fallen - but only ever at a distance.
Anyway, as they journey and adventure north, they advance quickly as PCs typically do - one dives deep into the gunslinging class they'd started with, another branches out into Shamanism, the dragon-blooded character begins transforming themselves into a massive primeval version of his race, and the engineer learns to create wonders he had only dreamed of before - including a set of wing prosthesis for his now massive dragon-blooded crewmate.
In their current adventure, they met a few members of Communion in the flesh, and were arrested shortly thereafter on their way to meet the local cluster. The crew were released the next morning, but the Communion "cultists" were not. They've launched an operation to free their new Communion friends, and are coming into the climax now: their ship just swooped in on where their friends are being held as high-level Heroes of the Empire bear down on their position and the city itself prepares to repel a new Brass Walker incursion! We play the conclusion in 8 days!
Here's the thing, though - Communion is the result of a psychic "gestalt" of poor, dispossessed, and dissatisfied people; who have joined their resources and skills together through a very powerful psychic link. What any one of them sees, remembers, or knows can be shared freely with any other; which in this world, means self-improvement - and self-enhancement - is absurdly easy to attain. They also offer shelter and nonjudgmental acceptance to the other poor and downtrodden of the Empire, which is a significant part of its population; so Communion spreads almost as fast as they can move without attracting attention. Many Communion members are content to keep their physical bodies mostly as they are; but just like our Dragon-blooded pilot, some enjoy becoming more physically capable. And when Communion members are physically threatened, these physically boosted members are the ones who rush to their aid. So every time someone threatens members of Communion, these "Brass Walkers" (the Empire's name for them) start showing up, things escalate, and eventually the Empire loses another city.
I am so looking forward to when my players realize they've not only met my steampunk, psychic Borg - but they've already begun assimilating themselves!