Aristotle's Poetics in 5 Easy Steps. Step 5: Theme
Step 5: Theme. I don't know if you've noticed, but something very peculiar is going on in Aristotle's Poetics. The audience brings a certain set of expectations about characters (certain people act a certain way) and the outcomes of actions (certain goals are achieved in a certain way). These are all based on the audiences' set of precepts about life (we all have them). Those act as the basis of the audiences' interpretation and expectations for the story. The story depends on those precepts to be intelligible, as well as for the twist ending to work. Remember, it's because of those precepts that the audience is surprised by the turn of events and reversal of fortune for the main character. It's that surprise that makes them wonder (which is the goal of the tragedy). The tragedy in order to work, must crush one or many of those precepts in order to be effective. The audiences' precepts about life are thus challenged by the story. To this point, Oedipus, then, isn't just about some tyrant sleeping with his mom and killing his father. It takes the audiences' belief that happiness is achieved by being a genius with absolute power and crushes it. In some ways, it's a massive critique of the Ancient Greek (if not our own) take on life: happiness is beyond you no matter how excellent your life is or seems.
Your story is an opportunity to comment on life. Your story is intelligible due to the audience bringing their precepts about life to it. The tragedy turns those precepts on their head. Given that the tragedy only works if our precepts of life are turned on their head, then it's your opportunity (and duty) as a writer to figure out which precepts of life are best turned on their head. For example, in the story "Of Mice and Men' Steinbeck takes our precept of the American Dream and shows that it's really a fantasy: it's something that does little more than keep us going through hard times.
When you design your story -- before you even write down a word of it -- start with determining your theme. Figure out which precept of life you want to turn on its head and make a statement about it. Maybe it's that the American Dream is a fantasy that keeps us going in the worst of times. Maybe its how we treat the elderly and the despair we cause them. Maybe its about fast food. Whatever it may be, make it your honest revelation you've had about life and write about that. Because when you write your tragedy and make your audience wonder about what happened, they're going to start wondering about the precept of life you just crushed, and then, maybe, they'll understand the revelation you've had about it and shared through your twist ending.
And then, maybe, you've successfully shared your revelation about life with the audience and made us all a little bit wiser.