In this fan fiction, perhaps there are new interpretations of the themes from 'Grapes of Wrath'. For example, the theme of struggle and survival in 'Grapes of Wrath' could be intensified in the WoFwalk world which might be full of more dangerous and unpredictable elements. Characters might have to fight not only against social injustice as in the original novel but also against the magical or otherworldly threats in WoFwalk.
Well, one interesting concept could be the way characters from 'Grapes of Wrath' interact with the unique world of WoFwalk. Maybe the Joads, from 'Grapes of Wrath', find themselves in the WoFwalk setting and have to adapt to its strange rules and creatures. It could lead to some really unique storylines.
Another concept could be the transformation of the characters' personalities. The harsh environment of WoFwalk might change the Joads in ways that we wouldn't expect. They could become more resourceful or more cynical. The fan fiction might explore how these changes occur and what impact they have on the overall story. For instance, Tom Joad might use his new - found skills in WoFwalk to lead a resistance against both the inhumane treatment from the 'Grapes of Wrath' society and the menacing forces in WoFwalk.