This one is really simple, I don’t think I’ll need a large wall of text to describe this one.
The player is a voice inside someone’s head, the objective is to stop the host from taking medication or seeking medical assistance. Your objective is to perpetuate your existence, but this can abstracted away from the player by making the player think that he/she is serving a different purpose. Say for example, the player might be a humble stranger that stepped into a fight to save a child (when really the child actually saved himself). Or maybe, the player’s continued existence improves that characters success in life, this option would still work even if the player is aware of the situation.
The reason why I think that this would be a good idea is that the player’s goals are actually being implemented as a story device and that the pursuit of the player’s goals augment the portrayal of schizophrenia. A lot of games that I’ve seen don’t give the player’s goals and motives implementation in the story. Most of the time the player is merely executing the will of a protagonist, with his/her own goals that are not on the same level as the player’s.
For example, in a game like Halo, the player’s motive would probably not be to kill the covenant and save humanity, but to have fun in executing the demands of the plot and game-play. In a multi-player game, the player’s motive might be to pwn some squeaky voiced 13 year-olds, rather than to actually complete the objective assigned to the character with clinical accuracy.
In order for this idea to work, there needs to be a strong emotional connection between the player and the schizophrenic character. This way their actions actually have genuine motivation behind them.