Your story's genre will influence the narrative and affect the way your characters act and interact with each other. For example, if writing a love story then you need to think about what kind of love story it is. Sad? Hopeful? Nostalgic? The pace of events in your story will need to match the emotional flow suitable to the mood, which frequently begins with the genre. Also, your character development will reveal different traits in a romance novel than it would in a murder-mystery story.
The short story writer Saki wrote about animals as a way to convey the beastliness of man, and Angela Carter used fairy tales as a way to express dark sexuality. Try writing a fable with simple language that represents a greater truth. You could write a story about the futility of war set at a family reunion, with the dysfunctional relationships and repressed emotions representing key ideas.
Roald Dahl, the celebrated children's author, wrote many short stories for adults that ended with an unexpected and macabre plot twist. This is a good way to improve your plotting skills and use of tension. The most intense psycho-thrillers do not rely on blood and gore but focus instead on the psychology of the characters. For your twisted tale, take inspiration from myths and legends as well as contemporary authors. The story "Lamb to the Slaughter" by Roald Dahl, in which a wife kills her husband and feeds the evidence to the investigating detective, is a great place to start.
To make your story original, try experimenting with the way you tell it. As you are limited by the brevity of the short story form, you are limited in the ideas you can use. Try writing loose but interconnected sketches to convey a set of emotions, like the way a dream feels when you try to remember it. Tell the story from an unusual perspective, such as a stationary object that observes the action but cannot participate.