Hnnn, my interest, it is piqued.
Original characters. We're looking at a well made and diverse setting, so let's do it the justice it deserves by making up our own stories, not just recounting what has already been.
Personally, the setting is ripe for a plot and layout similar to the show. The unrest, combined with the poverty on the rim, could easily lead to some freelancing work when one acquires a ship, legal or otherwise. Since there seems to be some interest leaning towards a luxury liner, this leads me to think that inserting some intrigue via the various crime organizations, such as those of Badger or Adalai Nischa, as horribly as I may have spelled that last one, would not go amiss, and might stir up some nice plots?
Aliens? Absolutely not. Firefly had a beauty in it's realism, in it's focus on the people, the politics, and the day-to-day, not on crazy, fantastic creatures or sciences, save perhaps the case of River. Aliens simply don't belong in a setting that didn't even both with Faster Than Light travel.
Personally, I would think someone lithe and violent, perhaps a simple card dealer to the guests, but a swift hitman for the cheaters and other unpleasant folk likely to frequent a luxury liner. In a way, opposite Jayne, but with a similar job.