Hey! It would be awesome for Sartosa to have some kind of shantytown behind them, or some kind of building to represent their homebase. Another idea for Vampire Coast would be to have the units on a pirate ship itself, with visible planking, railing, rigging, the ship mast, maybe ship superstructures or an anchor. I'll keep the ideas coming.^^
Edit 1: Somehow got an update on this, don't know why, but i got some more ideas!
Would it be possible to add Kraka Drak (the mod) to your supported list? A nice idea for their background would for the skybox to be like in Vermintide 2 in Norsca, with those chaotic northern lights. Don't know if this would be distingishable though, still, very fitting.