One possible alternate ending might be that Mr White is unable to prevent the entry of his son, returned from the grave, into the house. This would take the story into full-blown gothic horror mode, and leave little to the reader's imagination. It would be a more grisly, and probably less effective ending than the one that Jacobs actually opted for, which is much more subtle and thought-provoking. The fact that we do not actually see Herbert return, but only hear the door-knocking, adds a whole intriguing layer to the tale.This kind of subtlety is in the tradition of many of the best supernatural stories, where everything is not spelled out clearly, where more than one interpretation of events is possible, leaving room for doubt and intrigue.
The possible alternative ending suggested by gpane in Answer #1 is excellent and very suggestive. It almost cries out for someone to write such an ending to the story. When the reader visualizes that scene and imagines all that insistent knocking, he can't help glimpsing a mental picture of the person who is standing outside and wants to come in. Mr. White had to look at his dead son to identify him, and he knows how horrible the poor boy looked after being caught up in heavy machinery. The author inserts some brief dialogue towards the end of the story to remind the reader of what Herbert would look like--if that is really Herbert outside doing the knocking.
"Go and get it and wish," cried his wife, quivering with excitement.
The old man turned and regarded her, and his voice shook. "He has been dead ten days, and besides he--I would not tell you else, but--I could only recognize him by his clothing. If he was too terrible for you to see then, how now?"
So Herbert's face would be mangled beyond recognition and his whole mangled body would have begun to decay after being dead for ten days. If Mrs. White succeeded in getting the door open before her husband managed to make his third wish, she would see, in an alternative ending, a horrible monster standing there. What would he want? What would she do? Herbert would obviously want to move back in with his folks. He would be a living dead man. His own mother would die of fright or go out of her mind. His own father, having used up his last wish, might try to destroy his son by setting him on fire--and burning the house down in the process. There are many possible alternative endings based on the premise that it really is Herbert and that his parents really open the door for him.