In a terrible shipwreck, a man washes up on the shore of an island, barely breathing. But the island is not deserted; a native girl lives on the island alone, and it's been years and years since she's seen another human being. She finds him on the beach...
That's the basic setup, but can she speak English? How do they communicate? How did she get on the island in the first place? What will happen when they fall for each other? How will she react to being so close to a man after so long? How will he react, trapped on this island with a beautiful woman?
I'm open to lots of interesting variations, too: maybe it's a woman that survives a shipwreck and a native man who finds her. Maybe they're enemies at first, but are forced to work together to survive.
This idea is stolen with permission from RedEve - it was originally proposed as a one shot, but I think it has potential to be a longer story.