Great thread, everyone. Maybe it's run its course, maybe not.
It looks as if Underrail 2 / North Underrail is a strong contender for the next DLC, or it's in the works at some point.
Styg probably has the main campaign and story largely mapped out. So I just have some suggestions about the theme or gameplay.
North Underrail probably isn't all Rathounds and Crawlers like so much of South Underrail and it's caverns. It's said to be more built-up, upscale, advanced, etc...
So a number of people thinking about this have come to the same conclusion: maybe a lot of the quests would focus on intrigue. There's different, powerful factions there. If your character is potentially coming from Level 25+ (potentially), it's unrealistic to combat a bunch of trash mobs. And there may be fewer low-level baddies there anyway.
But the Protectorate, the corporations of Hexagon, Dis, the Free Drones are likely there. And they all have clashing, competing interests. The game, as stated earlier, could focus more on investigation. Finding out who is worth supporting, and putting your weight there. Fewer, but more critical combats, with more sophisticated enemies, punctuating what we learn in the storyline. More hidden boxes, secret passages, sewer entrances, hacking computer logs, intelligent robots, careful conversations with people and AIs. Silent assassinations. Disabling critical systems so that your allies can attack (which you can participate in).
Of course, there'd still be lots of room for gang fights, like we had with Gorsky's Zone Rats. I'm thinking a more cyberpunk / Zaibatsu situations. And with the corporate espionage, more sneaking / shooting through horrible back-areas to get there. And there's still be slums or run-down areas, muggers, and so on. Restoring old industry, having to clear out the areas of newer, higher-order monsters. Finding incriminating logs from Biocorp, etc...
And that gets me to a major thematic element. In Underrail, you are a puppet, first of Tanner, and then basically Six, or at least your situation in the DC. It wouldn't surprise me if you became somebody's puppet again in North Underrail, either a faction,* Six, or both. Sure, you get to do something satisfying, like punching Tanner in the face, but there are bigger fish to fry there.
But what if you could get loose from your master? But that's not an official part of the main quest, or quest sections involving these puppet masters. Instead, if you read the clues, read the hidden logs no-one knows about, you can find ways to escape your bondage. And become a free agent, or whatever? What I am saying is, an observant player could break away from certain 'mandatory' quests to pursue their own (or SGS) interests. Just as an 'answer' to a thematic element of Underrail.
Anyway, those are my ideas. I think there's space for greater intrigue and faction-choosing in North Underrail, where interpersonal skills could become a lot more important. And the vents... maybe someone should tell your character, "Here in North Underrail, vents aren't neglected as much." They're part of critical high-quality air purification, and people are watching them / in them. There would be quests to sabotage / repair them. Maybe a large part of the game would be like the Core City quests, but deeper.
* Speaking of factions, there are always consequences. Joining Oculus is very rewarding, and there are no downsides. But what if they start putting the screws to you in North Underrail, demanding you go on quests for them? Sure, they'll reward you, and you're their "friend," but if you refuse they will try to off you. Like that sword guy at the end of Expedition.
The same applies to the Protectorate and the Free Drones. Right now, there's not much advantages to my characters being neutral (even though that is often what they choose), but what if that allows you to go to more places in North Underrail, that would be barred to 'partisan' characters?