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.
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.
Friendship is a very common theme. Their unique bond, where Shrek is the big, gruff ogre and Donkey is the chatty sidekick, is often explored in different ways. Another theme is adventure. Since they often go on adventures in the original movies, fanfics might continue those adventures or create new ones for them. Also, acceptance is there, as Shrek has to accept Donkey's presence and Donkey has to accept Shrek's ogre - ish ways.
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.
One possible plot could be that Donkey Kong and Mario have to team up to save the Mushroom Kingdom from a new, powerful villain. They combine their unique skills, like Mario's jumping and Donkey Kong's strength, to overcome various challenges and defeat the bad guy.
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.
Bowser could still be the main antagonist. He might see Donkey Kong and Mario teaming up as a huge threat to his plans of taking over the Mushroom Kingdom. So he devises even more devious schemes to stop them, like creating new types of minions or using powerful magic artifacts.
One popular 'Shrek x Donkey fanfic' is 'The Unlikely Bond'. It explores their friendship in more depth, showing how they support each other through various adventures in the swamp. Another is 'Shrek and Donkey's New Quest' which takes them on a journey to find a magical flower that can make the swamp even more beautiful. There's also 'Donkey's Insights' which is mainly from Donkey's perspective, sharing his thoughts about his friendship with Shrek.