My impression :
I was never really a fan of factions lacking activity - such as commerce, supplies.. The Quest for Resources - I think the Black Market & Corporate is under used in commerce & supply activities.

That being said :
I think you are heading with the first two ideas - to interrupt or establish commerce / supply routes as opposed to the 'kill everybody in sight' missions.

As for the Fates of the Slaves, exactly what the Commonwealth and Corporate do with them really should never been so easy as "releasing them" ( The Sister were always perfect ). ( If Corporate / Civilian stations )They should be sent to or be taken to a medical station. (If Military) to the Hospital Ships or some Military station for 'Classified' or unknown fate.

I never liked that the Sung throws everything at the Hauri Temple as an 'End all' event when they never really have skirmishes in the systems.

I was always confused as to why the Sung automatically regard your ship as a hostile entity. No idea how this could be implemented, but I think a method to work *with* the Sung as opposed to the Commonwealth ought to be an option. I get that for balancing issues the Commonwealth need to be stronger and fighting them would be more or less suicide until you reach end game levels of equipment, but adding the ability to be a bad guy should be there in some capacity.

I'd also like there to be deeper interactions with the slaves liberated from the coffins themselves. I know this isn't implemented, but maybe having an actual crew that complements your ship? My headcannon was that your main character was actually an implicit crew that had the pilot, some engineers, and someone with a Domina connection. Having crew dynamics would definitely add more dimensions to gameplay.

But as for Sung missions themselves, have a certain high value target (politician, religious figure, CEO, etc) be a missing slave that needs to be found amongst one of the Sung Citadels. Once freed, they can give out some kind of quest.

The Sung are slavers to an extent that feels pretty squicky to me. As I understand it, they don't just steal physical freedom but also free will, reducing a person to a computer chip that is forced to do what it's told. I for one would be very uncomfortable with siding with them. If you want to play as a bad guy, there's plenty of space for exploring that with Domina -- handing over a bunch of slaves to her isn't an obviously good deed, even if you aren't aware that she doesn't exactly have the galaxy's best interests at heart! There's also the Ares conflict of course as a place where you get to choose sides.

Anyway, as for the underground railroad. Let's just establish some ground rules here:

- The slaves have to be extracted from Sung control somehow. There's a number of ways this could be done, e.g. intercepting Sung shipments, daring raids on Sung stations, electronic warfare vs. Sung ships that allows some slave pilots to overcome whatever control the coffin exerts on them (or perhaps some pilots are just able to do that independently, due to flaws in the coffin's programming or something).

- Once out of immediate Sung control, they then have to make their way to safety, while being pursued by the Sung who want them back. The underground railroad would be a network of station-based safe houses and smugglers or other "deniable" transport ships that have a chance of not being intercepted.

- Once at St. Kat's you'd think the slaves would be more or less home free. The remaining work would be mostly rehabilitation: getting them out of their coffins, fixing up their bodies, addressing mental trauma, and acculturating them. It's not clear to me how useful the player can be in this step. However since the Sung do have a presence at St. Kat's, I suppose they can stage their *own* daring raids to "retrieve their property", much like was done in the USA in the 1800's, when there were extensive laws even in free states about whether you were allowed to try to stop a mob from taking slaves that had escaped from them.

I'd be inclined to suggest the following mission series:

- The underground railroad has found a particularly valuable slave, who has important knowledge about Sung ties to something important (Domina/Orcus? The Ares? The Huari?). They've managed to get the rescued slave to the St. Kat's gate, but the Sung are staging a final attempt to stop the freighter, so you need to go escort them in.

- Once the freighter makes it to St. Kat's, there's a problem with rehabilitating the slave. The Sung have updated their "shackling" protocols which join the slave's thought processes with the computers in the coffin; the two can't safely be separated. If the railroad operators could get their hands on a slavemaster's keybook, they could reverse-engineer the updated protocol. The player is sent off to raid a Sung station in the system and capture this item.

- Having liberated the slave (their thought processes, if not their bodies just yet), they have urgent information to impart: the Sung are staging an attack on St. Kat's to drive political change. They're using some captured Huari ships in an attempt to further ostracize the Huari. Or maybe they're pretending to be some other faction (Dwarg?), but the point is that they're trying to turn the Commonwealth against that faction for their own ends. The player needs to intercept the attack force and destroy them before they can damage the ring arcology or any civilian ships.

Oh, and incidentally -- the above is at odds with the current behavior when you release slaves on a Commonwealth station, as that implies that they can just get out of their coffins and walk off, no muss no fuss.

Additionally -- missions to help the underground railroad could be decent "filler" missions in the Ungoverned Territories. Sort of the UT version of Korolov missions.

Should be noted that the mission arc shouldn't break "No Backtrack" conduct on original scenario (SOTP). I never do No Backtrack, but I think some people may want do the challenge with also doing the mission arc. Huari mission arc is also break No Backtrack, but fortunately they counted as hidden mission so they can easily ignored.

So I suggest the mission arc begin in St. Katharine systems and finish at Jiang Star. The first mission is to destroy the slaver camp or intercept Sung transport or protect the freighter to rescue the key character as main quest giver. Perhaps players need to reach certain rank on Militia service to accept some near-end mission, which relatively similar with Fleet mission arc. It would great idea if we have introduction to COMSOC and player meet Stormhound again in this arc.

Oh, is the arc allowed to leave St. Kat's? I thought the goal was to do an entire mission arc in a single system. My mistake.

I second the suggestion of a "pro-Sung" option. I understand what they are doing is morally wrong, but the main gameplay loop of Transcendence is a pseudo-genocidal rampage that leaves entire systems decimated. The player essentially uproots their entire life to go on a journey to reach the galactic core by any means necessary and is completely capable of dealing in the slave trade- I see no reason for why they would hesitate to deal with the Sung other than purely pragmatic ones.

That being said, I remember a mentioning of a reformer faction in the Sung that wants to replace the slaves with vat-grown biological neural systems. I very much like this idea as the concept lends well to the same system used in the new Raisu Mission and could easily be lead into from the Underground Railroad missions.

As an example of execution, Set up a new station (or just another wing of the arcology) dedicated to the rehabilitation of former slaves in St. Kats, one which sends you on a mission to destroy a slave camp/sung transport in system. You are introduced to a mission handler and a new temporary wingmate, a former slave stuck in their coffin that chooses to fly against the slavers. As you progress with the Underground Railroad, you are introduced to the Huari and soon after a mission string begins where said wingmate believes the Railroad is compromised by the Sung.

Soon it becomes obvious that the Railroad is not compromised at all, but rather is secretly in contact with a faction within the Sung led by rogue Deva which concluded it would be more efficient to replace slaves with the aforementioned vat-grown biological neural systems. Said faction provides much of the intel, working to convince the opposing Devas by artificially raising the risk/cost associated with slaving through supporting the Railroad. Because of your wingmate's investigation, the Huari are now aware of such cooperation, and are threatening to stop supporting the railroad unless the player attacks a reformer station. Now it becomes a choice between allying with the Huari and waging a war of extermination against the Sung, leading into their arc, or siding with the reformers against their slaver counterparts. Both could end with a massive battle against the Dragon Slaver, or going the reformer route could give access to a Sung Cyberdeck or something.