"part of a series of Passives that have been incomplete thus far."
Honestly not sure what the new gameplay mechanic will be. If its Dogenza or Masochisim/Sadism based skill, what are the incomplete series of passives Rem is refering to?
New art assets for existing CGs makes me think of stuff like Hairstyle/color, Piercing/Tattoo, Uniform..
There are edicts about about upgrading the uniform, with descriptions like 'I didn't expect the new waitress outfit to be so flimsy'. So uniforms would be my best guess.