Final answer:
If Mrs. White had opened the door before the final wish, possible outcomes include facing a horrifying version of their son, Mrs. White's mental and emotional destruction, or revealing nothing, enhancing themes of fate and the unknown.
Step-by-step explanation:
If Mrs. White had opened the door before her husband was able to make the final wish, there are several possibilities that could have unfolded in the story "The Monkey's Paw". Firstly, the consequence of foreshadowed horror might have been realized, as whatever stood at the door could have been a disfigured or unrestful version of their son, leading to unimaginable terror for the Whites.
Secondly, the encounter might have destroyed Mrs. White mentally and emotionally, considering her desperate state of mind prior to the knocking. Thirdly, it is also possible that opening the door would have revealed nothing there, furthering the theme of supernatural ambiguity and the psychological torment of the unknown.
Each of these scenarios would have altered the story's outcome significantly, emphasizing the cautionary tale about meddling with fate and the irrevocable nature of death.