I had the idea of a 'Spirit Playbook' where you play as a Hollow. Anyone is free to take this idea as their own. Here's what I'm thinking:
They're a living body without a spirit. Their former human feelings, interests, and connections are only dim memories (like Hulls). They are generally emotionless, dull, dim-witted, and easily controlled. As a result they're immune to supernatural terror and can't have trauma because nothing really fazes them. In addition, as a result of being crippled emotionally, mentally, and spiritually, they're dependent on others to survive, and so the only way for them to remove stress is for someone else to use a downtime action to take care of them (make sure they have food and are eating, see how they're feeling, and so on).
They could have a fairly short stress track and instead of trauma they could have a more temporary mechanic where they become increasingly helpless. Such as wandering off, becoming fixated on something, staying exactly where they're left when separated, or otherwise not acting according to common sense or self-preservation. The idea being that someone needs to keep an eye on them and guide them by the hand when necessary. These would be 'temporary' in that maybe they could be removed through downtime action, where a character helps and gives them instructions so they won't be as helpless in the future. All in all, similar to the Wear mechanic from the Hull Playbook.
Their XP triggers would also be similar to the Hull Playbook, and aimed towards rewarding roleplay of the above concepts. They mark XP if they suppressed or ignored their former human beliefs, drives, heritage, or background; if they struggled with issues from their innate helplessness; and if they followed a command despite difficulty, danger, or detriment.
The charm of playing a Hollow would be in the slow transition from being a helpless and emotionless doll to becoming your own person (perhaps even recovering the memories and personality of your old self). Their traits could be connected to Demons and the lore surrounding the Church of Ecstasy, who believe that the purest beings are those without spirits and that there might be a way for a human to undergo 'ascension'. Giving them an elemental affinity and access to specific demonic powers, like Remote Viewing, would be particularly interesting.