The Flash's original cut ending would've avoided most of the major criticisms levied at the movie by critics and audiences - except one potent issue.
The original cut ending for The Flash could have fixed almost every criticism of the movie, aside from one controversial issue. The Flash did receive some notable positive critical reviews, separating it from more universally lambasted superhero films like Suicide Squad. However, this isn't to say that the movie was without fault, as considerable criticisms have cropped up regarding the visuals and narrative of the superhero installment, as well as the decision to make cameos by digitally recreating deceased actors.
  
Later reveals suggested that some issues were the result of the
    movie's plot having to be changed to fit best with the impending new DC
    Universe. Perhaps the largest of these changes was the news that The Flash
    had three potential endings - one where Keaton's Batman and Calle's
    Supergirl appeared in the final scene, one wherein Cavill's Superman and
    Gadot's Wonder Woman appeared alongside Batman and Supergirl, and the ending
    that made it into the released cut of the movie. While changing the ending
    makes sense for the overall universe, the original ending would've fixed all
    but one of the main criticisms The Flash has faced.
  
 The Flash's Original Ending Fixed Every Complaint But The Cameo
      Criticisms
    
Some of the main complaints raised at The Flash are in regards
    to the stories of Michael Keaton's Batman and Sasha Calle's Supergirl, as
    both have their personal arcs end somewhat jarringly with the revelation
    that neither can be saved from the battle with General Zod, no matter what
    steps the Flash duo take to try and make sure they come out of the conflict
    alive. These complaints play into other issues raised with the movie,
    including awkward pacing and a feeling that the Dark Flash reveal and
    storyline was somewhat wasted. 
  
Had Keaton's Batman and Calle's Supergirl reappeared in The
    Flash movie ending, it would have recontextualized both their stories and
    the movie itself in a way that would have at the absolute least lessened
    these complaints. Bringing the characters back prevents the final scenes
    with the actors closing with some awkward death replays, and also makes the
    film's plot more about the team than the two versions of Barry Allen,
    meaning that elements that seem weird in a more Flash-focused movie instead
    are more clearly because of the divided attentions of the story.
  
That said, this of course still wouldn't fix the criticisms
    surrounding the posthumous cameos that appear within The Flash, meaning one
    of the most pressing points of contention regarding the movie wouldn't be
    changed by this alteration. This means altering the original ending
    ultimately can't be held responsible for the reaction many have had to the
    overall story, as all three endings would've still seen The Flash be
    criticized for insensitive handling of its multiversal scenes.
  
 Why The Flash Changed The Original Ending (& Why It Hurt
      The Movie)
    
The Flash's original movie ending was changed by DC Universe
    figureheads James Gunn and Peter Safran, seemingly to ensure the transition
    from the DCEU to DC Universe went smoothly and with minimal audience
    confusion. Though the initial ending could have fixed a lot of snags with
    The Flash as an individual movie, it would have created more for the overall
    franchise by suggesting Michael Keaton's Batman and Calle's Supergirl would
    be definite fixtures of the upcoming new DC superhero squad, which doesn't
    appear to be the case. As such, altering this ending does appear to be
    something of a necessary sacrifice - although one that may prompt more
    divided responses to The Flash than perhaps was initially expected. 
