The short film is a unique medium, so there are certain things to consider when coming up with an idea for one.
A Brief Moment of Celluloid
Short narrative films are a peculiar art form because the normal allowances that you have in filmmaking are restricted. Your ideas have to be much more focused and concise, often creating a problem that can be resolved in a few minutes of film time. When coming up with a good idea for a short film there are a few things to keep in mind that will help you make it work.
The first thing you have to consider is that there has to be simplicity in the storyline. This does not only mean not creating complicated plot points, but also that the number of locations, camera angles, and cut aways need to be somewhat limited. The reason for this is that you will likely have a short period of time to film it and a small budget to get it complete. If it is story oriented then too many complexities will be hard for the audience to connect with in a very short period of time.
You may want to think of a small device, whether based in the story or aesthetic, to focus on. This is the kind of thing that would work in concert with the overarching film in a feature but can be the central theme of a short film. This can be a visual experiment, use of extended long cuts, or anything else that would be a small set piece in a longer project.
Use Your Limitations
Consider the limitations of the short film as its strengths and try to maximize them. The ability to go from the beginning to the middle and the end is hard in a short film so try to think of a story that can be summed down to a sentence or two. This is going to end up working better for the medium you have then trying to break the limits of the short format.
Think about a storyline that is highly visual rather than dialogue based. Good dialogue relies on extended periods of time to get across a realistic story progression. You do not have that time in your short film, so you should go through something that “shows rather than tells."
You really have to consider how to compact a great film experience into just a few minutes. Itemize the theme you want to use with the characters and plot structure you connect with and go from there.