I think that an alternate ending for the story could focus on Mrs. Salmon.  The idea of the eyewitness, someone tangential to the event in question, becoming the center of the story could be compelling.  Mrs. Salmon ends up becoming a critical element of the narrative as she is unable to distinguish between both twins.  She leaves the courtroom as a broken woman, to an extent.  The poise and certainty with which she was called as a witness was undermined in the course of the trial.  This "extraordinary" condition was only enhanced by the fact that she witnesses the death of one of the twins outside the courthouse. She is left with the sense of complete fragmentation.  Which one, the murderer or the innocent one, died?  What role did her testimony play in putting these horrific chain of events into motion?  As she witnesses the dead body, the narrator asks how it could be possible for Mrs. Salmon to "sleep at night."  It is here where I think that she could be a great focal point for an alternate ending that focuses on her life after the trial and how she functions. From being a tangent, she becomes center.  From the margins she moves into the crosshairs of central focus, a reason why detailing Mrs. Salmon after what she witnesses could make for an interesting alternate ending.