How do you write a short story based on the same ideas and issues presented in the Arthurian romances?
The Arthurian stories, like most oral-derived texts served to encapsulate the ethical and religious values of their culture. As you seek to replicate the value system of the stories, your first major decision will be whether to transpose them to a contemporary setting or whether to keep them in their period.
One possibility would be to base a story on the idea of the return of King Arthur when England is at the time of its greatest peril. You could set it in World War II or use a theme based on a contemporary issue (e.g. Arthur returns to save England from a terrorist plot)
Another possibility would be to retell the story in its original setting but from the point of view of a minor character such as a woman or servant.
In either case, you might construct your plot to revolve around an ethical choice.