Got an idea and I’m gonna run with it.
You’ve been locked up for tests in a top-security government lab because of your ‘special’ abilities, one of which is being able to leave your physical body in ethereal form. You want to escape, obviously. Your physical and ethereal forms have different abilities, so both are needed to get past the various locked doors, hazards and guards etc. However, and this is the theme part, your life decreases when you’re away from your physical body so you must limit time spent in ethereal form.
Sounds daft and complicated when I write it all down like that but it should be straight forward enough in game form.