Law of closure, interesting would be to experimentally check what the limits are of the law of closure and that for different things: Text, symbols, faces, known and unknown things, explore under what condition the law of closure fails and delivers false results. Does the law of closure goes beyond visual sensory input? Hearing?
Principle of Prägnanz, could be it is kind of compressing, sensory input to a kind of unique fingerprint, from which the complete thing can be reconstructed fast and as easy as possible.
An interesting example of non visual gestalt theory is for audio sensory input: morse code reception where the spacing between dots and dashes and grouped together -.. is d and -.. . is de. When the morse code is sent sloppy all understanding fails.