What about a theme about the four elements (fire, air, water and earth)?

You could pick any of the four to take a center role in the story.

You'd get a bonus for:

Combining all four
Not using a fantasy setting
Not using (typical?) magic
Letting a person or animal embody an element and personify it
Playing with the stereotypes of the elements (making reckless fire suddenly tame, or making sturdy earth actually fragile)