Sounds like a cool concept. Try layering a faint, low‑frequency hum that only pops out when the mix thins a bit, like a secret undercurrent. Add some subtle spectral shimmer—maybe a low‑pass filtered synth that rises and falls in a slow sweep—so the listener notices a shift when they’re really listening. Keep the changes very gradual, almost imperceptible, and use a touch of reverb that expands with the scene to hint at another dimension. If you push the mix too hard, it’ll feel like a gimmick; just let the layers breathe and let the sound itself reveal the hidden world.