One idea could be that they face magical challenges together after getting married. For example, an old enemy of Timmy's tries to use their new union to gain power in the fairy world. Another could be that they have to deal with the cultural differences between humans and fairies in their married life. Maybe Trixie discovers some unique fairy traditions that she has to adapt to.
Timmy, who has always been a bit of a dreamer and a kind - hearted boy, might become more responsible as a married man. He has to think about Trixie's needs and their future together. Trixie, on the other hand, could become more empathetic. Maybe she starts to understand Timmy's connection with the fairy world better and becomes more involved in his magical side of life.
The theme of heroism is also present. Timmy becomes the hero in these stories, even if he gets hurt. His actions inspire Trixie to be braver herself. And there could be a theme of redemption, especially if in the normal story arc Trixie has been mean to Timmy, but now she has a chance to make it up to him as he's injured because of her.
One possible plotline could be that Timmy and Trixie are in a magical forest when a group of dark fairies attacks. Timmy uses his fairy - godparent powers to shield Trixie from their harmful spells, getting hurt in the process. Trixie then has to find a way to help him, like searching for a special healing flower.
I'm not aware of any official fanfiction specifically about Timmy Turner and Trixy Tang in such a situation. Fanfiction can cover a wide range of ideas, but this might be a rather unusual or even inappropriate concept for the Fairly OddParents universe.
A 'mlp trixie sad fanfic' might be about the character Trixie from My Little Pony in a sad situation. It could explore themes like loneliness, failure, or disappointment. For example, the fanfic might depict Trixie being ostracized by other ponies in Ponyville after a failed magic show, which makes her sad and question her self - worth.