One way to improve a screenplay is to find a way to look at it from another viewpoint - whether the story is to be optimistic or pessimistic. If we take the notion of optimism versus pessimism and look to Aristotle, we can come up with a truly original ending for our scripts. Aristotle defined a "reversal" as being a plot change by which the action veers round to its opposite. According to Aristotle, the best reversals are caused by the main character's recognition of something that causes the reversal, so it doesn't come out of left field, that is, it must be subject to the boundaries of probability or necessity. The reversal arises out of the recognition of something that could have been seen before, but was not. This is where we reach those edges of boundaries that will help you find that original twist you were looking for.
One of the cornerstones of my method is the use of the main character's personality to drive the plot. An example of recognition and reversal occurs in The Line Of Fire. This is Frank Horgan's (Clint Eastwood) recognition that the field office number is an anagram, and that unraveling a different anagram can reveal the secret of the would-be assassin's identity. We have been directed towards a pessimistic ending - that is, it's been set up in Act I that Frank won't catch Leary (John Malkovich) in time, and Act 2 has furthered this expectation, but this recognition of the anagram is what allows for the reversal to create the opposite outcome in the third act. Frank has gone from being pessimistic to being optimistic.
Another example of recognition and reversal occurs in The Usual Suspects, in this case, the reversal is of the audience's expectations. It is the audience's optimistic belief that "Verbal" (Kevin Spacey) is innocent that allows us to enjoy the reversal when he's revealed by his limp to be the perpetrator, and not the victim. We have gone from being optimistic to being pessimistic.
Here's the exercise:
First re-read your script or outline and decide what ending is suggested by what you have written or designed. Is it optimistic or pessimistic? Ask yourself whether or not your ending is predictable, that is, does it end the way it's been suggested in the first two acts of the screenplay or outline.
Imagine that you are your main character. You are going to write in the first person as that character, and describe from their point of view a different ending than the one you'd planned for Act 3. Remember not to worry about the execution, but have fun imagining another outcome. The first line should read, "I had no idea that..."
If you were writing as Frank In The Line Of Fire, you might write something like this, " I had no idea that I would ever get another chance to catch Leary. I was the same loser I had always been. As I headed for the taxi, the sick taste of failure in my mouth, the young smartass agent rattled off the number for the field office I was being sent to. Then it hit, me - I suddenly knew how to find Leary! It was the old feeling of sureness I hadn't felt since I blew things with Kennedy, and I had no reason to believe in myself, but I had nothing left to lose..."
What you've done here is to allow the character to "tell" you how they made the recognition so that your ending can have a strong reversal! It can almost feel like Magic!
Now ask yourself what the opposite ending might be, but don't concern yourself with how you will make it work just yet, just imagine it will be fine. For example, the predictable ending of The Usual Suspects is that "Verbal" is not "Soze", but the opposite ending and reversal of the audience's expectations comes from his realization he is getting away with it and as a result, stops concealing his limp.
When you re-read what you wrote, did you get an unexpected and clever ending? Is it one that you would prefer to use? If so, don't worry yet about how to make the reversal and recognition happen, but commit to making the change in your screenplay. If not, go one step further and ask yourself what is the most unlikely ending? Sometimes that will provoke a new insight. Otherwise, consider that the ending you have is just right, and commit to the best execution possible.
Let's assume you found a new ending. You can now go back and reverse engineer how to make it happen. We do that by revisiting the events of the preceding two acts and finding where the assumptions have been made. In the Line Of Fire, Frank is shown to lack confidence in his own judgment, so by giving him a moment of recognition when he has a gut feeling that the anagram is the key, his confidence is restored. In The Usual Suspects, it's "Verbal/Soze's" supreme confidence that both carries him through and then gives him away at then end.
To summarize: By moving from optimism or pessimism and applying Aristotle's simple principles of reversal and recognition, it's possible to come up with a great new ending for your screenplay.