As I think more about these people, the more I want them to be people who have had no previous connection to Aether, Galactic or anyone like that. These are the ideas that appeal the most to me:
Ultra Space citizens. I'm assuming as of right now that they're wearing masks, but it could be possible that they're not masks and that they really do look very different from humans. I'm not sure why they'd be relevant enough to be on the poster if they'd only show up in Ultra Space, so if this train of thought is true then I'd probably expect some of the people to have fallen through the wormhole and, similar to the UBs, want to get home. Maybe they wouldn't harm a Faller as such, but rather simply be drawn to them.
Fallers. Since this idea was touched on in SM but never fully explored, I'd love it if these people were from a different universe and were trying to find a way home while making a living in this universe. They'd be characters with multiple dynamics - their job they've taken up in the new universe, their background before getting lost in a wormhole and how they've been trying to get back.
People looking for the UBs. Whether it be for protection or capture, I think it'd be fascinating to see a smaller organisation that works for the same destination as what Lusamine wanted, but with a completely different goal in mind. As protectors they may be what Lusamine claimed to want but inevitably became something much darker, and as capturers they could be more akin to the main protagonist in what they desire.