Right - that's a tough one. It's a lovely song, but with something this sparse it makes decisions harder.
It sounds as if everything is pushed way back - like everything is playing from the back wall of the room. You can't have just the vocals forward on their own - something else needs to be filling the space in the middle - both in terms of the frequency and the depth. It's not necessarily additional instrumentation, just some more aggressive placement of the synth or drum parts.
Not sure if I'm making any sense - but hopefully you get what I'm trying to say.