Jealousy is another theme that could be explored. The other children are jealous of Margot's memory of the sun. In a fanfic, this jealousy could be developed further. It could lead to more complex interactions between the characters. For example, instead of just locking Margot up, they could try more subtle ways to undermine her because of their jealousy.
One significant theme in a 'All Summer in a Day Fanfic' could be isolation. Margot is isolated in the original story because of her different experiences. A fanfic could further explore this isolation, perhaps by showing how she copes with it when the other children shun her. It might also show how her isolation affects her view of the world around her.