The Forgotten already feel a bit like Pirates, I think. Since their methodology involves Pillaging/Stealing and running. If anything, I feel like they should get a Sea Unit to go full Pirate.

For a Flying faction, I think you want to play up their mobility potential and speed. But probably avoid having them collect something like the Allayi, as that would make them too similar. Something that occurs to me is the opposite of the Allayi a faction that gets only a very small Penalty for expansion, but in exchange their cities have to be very small. Like 5 Districts max. So they have to map all of Auriga to guarantee the best possible places for the places with the most anomalies where these cities could thrive.