Since it's a 'elizabeth is not a bennet fanfiction', it could be a very interesting exploration of identity. In many traditional stories, Elizabeth Bennet is very much defined by her place in the Bennet family. However, in this fanfiction, if she's not a Bennet, it might mean she has a different social standing, different goals, and different relationships. Maybe she was adopted into another family and now has a different set of values. Or perhaps there was some sort of mix - up at birth. The possibilities are really endless when you start to think about how this one change can affect the whole story world. It could also be a way for the author to comment on the importance of family name and heritage in the original story, by showing what happens when that connection is severed for a key character like Elizabeth.
Well, 'elizabeth is not a bennet fanfiction' might imply a departure from the normal Elizabeth Bennet character we know. It could be a creative take on a story where Elizabeth is not considered part of the Bennet family in some alternate universe or plot. For example, she could be an outsider who has a unique connection to the family that is not based on blood or the traditional family ties. This could lead to all sorts of interesting plot twists and character developments, such as new relationships being formed and old ones being re - evaluated.
I'm not entirely sure specifically without more context. It could be a fanfiction where Elizabeth has a different identity or relationship to the Bennet family than in the original work. Maybe it's a story that subverts the typical view of Elizabeth within the Bennet family dynamic.