There are countless 'he said she said twilight fanfiction' out there. Some might explore the idea of a new vampire or werewolf entering the scene. Say, a new vampire arrives and 'he said' he has information about an ancient vampire law. 'She said' (in this case, perhaps Bella) that she doesn't believe it's relevant to their current situation. This leads to a story full of mystery and discovery as they try to figure out if this new information is important or just a ruse. Fanfiction often takes these simple concepts of 'he said she said' and weaves complex and engaging stories within the Twilight framework.
One interesting 'he said she said twilight fanfiction' could be about Bella and Edward having a miscommunication where 'he said' one thing about his vampire nature and 'she said' something completely different in response, leading to a rift in their relationship that they have to work through. Another could be a story where Jacob and Renesmee have a similar 'he said she said' situation regarding their feelings for each other and how it impacts the whole vampire - werewolf dynamic in the Twilight world.