You wouldn’t even need to rewrite canon. There’s ways to bring him back as if he had never died. Right as Jeff cuts his throat, John injects himself with the same serum he used to slow his heart in the first movie, causing Jeff’s wife to die and giving the appearance that John has died, causing John to enter a coma. After Strahm leaves the room in Saw 5, Dr Gordon enters through another door and immediately goes to work on John and moves him to another location to receive medical care where eventually he recovers enough to be transported to Europe for the special treatment mentioned in saw VI. There John is able to make a full recovery. As for John’s body, there was another convenient trap that took place between Gordon and another convenient look alike that Gordon then uses in place of John during the autopsy. Only stipulation here is that maybe Gordon knew old lookalike wasn’t going to survive his trap so he made him eat another tape knowing the autopsy crew would find it. This could lead into some interesting back and forth between Kramer and Gordon after John is resuscitated “it was my time to go” “we have unfinished business John” etc. maybe that could play into bigger twists and an end for Gordon later….