I remember, around the time the third film came out, reading that several Mt. Doom endings had been filmed, although I have no solid evidence to back up whether this actually happened. For one of the big things I’d love to change would be to have Gollum’s demise as it is in the book. I appreciate PJ wanting to make the ending a little more Hollywood cliff hanger-esque (quite literally), but I adore Tolkien’s ambiguous, even fatalistic, end.
If any extra footage does make the light of day then I’ll happily integrate it into the finished product but in the mean time I’ve made a start to the editing and am so far very happy with the progress.