Variations of Mirror
This most essential exercise has many variations.
Three Way Mirror is a good problem to face once Two person mirror is solved. There are other variations. Who Started the Motion warm-up game, Mirror/Sub-Teams, Fun House Mirror, where each reflection must exaggerate one or more body parts. i.e., large hands, small head, big feet, tall, squat, etc.
More advanced mirrors are Mirror Speech and Mirror Emotions with Who, What and Where.