A theme might be the journey of acceptance. Since they are such different creatures, the fanfic could focus on how they come to accept each other. Maybe the dragon has to learn to see the value in the donkey's simplicity, and the donkey has to overcome its fear of the dragon's power. There could also be themes of friendship or even an unlikely alliance, where they team up to face a common threat despite their differences.
Once upon a time in the Shrek universe, Donkey and Dragon became the best of friends. Donkey, being his chatty self, would constantly tell Dragon about all the adventures he had with Shrek. Dragon, with her big, fiery eyes, would listen intently. One day, they decided to go on an adventure of their own. They explored a far - off land filled with magical creatures. Donkey was a bit scared at first, but Dragon's presence made him feel safe. They found a hidden treasure chest filled with golden eggs. They returned to Shrek's swamp as heroes, and everyone was amazed by their tale.
I'm not aware of specific 'donkey x dragon fanfic' stories. However, you could try searching on some fanfiction platforms like Archive of Our Own. They have a vast collection of fan - created stories, and you might be able to find something related to this rather unique pairing there.
They could go on a quest to find a magical flower that can make anyone speak in rhymes. Donkey would be excited about this as he loves to talk, and Dragon would be there to protect him from any danger on the way.
Friendship is a very common theme. Just like in the movie, fanfictions often explore how the unlikely friendship between the dragon and donkey grows and deepens. For example, they might face challenges together and through that, their bond becomes stronger.
Mario is often a popular choice. Since both are Nintendo characters, it's easy to imagine crossovers. Mario's acrobatic skills and Donkey Kong's strength could create some really fun scenarios.