Simple categories matching featureless:

• area,
• surface,
• structure,
• terrain,
• object,
• obstruction,

Members with property featureless:

• white_area,
• heart_wall,
• wall,
• frame_object,
• flat,
• sheet,
• skin,
• frame,
• bland_rock,
• windows,
• stucco,
• body,
• black_mirror,
• desert,
• plain_wall,
• clean_wall,
• gravel_desert,
• white_wall,
• flats,