The orginal ending for "The Open Window" revolves around the surprise of the unexpected return of Mrs. Sappleton's husband and two younger brothers. If you were to rewrite the ending, you could take the story in several different directions:
1. Assume that Vera's story is true.
Rewriting the ending where Vera's story is true and the returning figures are ghostly images would make the story have a much darker, fantastical feel to it.
2. Vera gets busted for her wild story-telling.
Wouldn't it be entertaining to see Vera get caught in her lie? It would be fun to create an ending where Framton Nuttel pretends to buy into her story, only to fake a heart attack to teach Vera a lesson.