The combination of two very different universes, Chowder's and My Little Pony, makes it interesting. It's like a crossover of different worlds of food and magic.
The fanfic 'Chowder in Ponyville' is also worth checking out. It details Chowder's life as he sets up a little food stand in Ponyville. The story delves into how the ponies react to his strange yet delicious food. There are also sub - plots about Chowder making new friends among the ponies and learning valuable lessons about friendship and acceptance, which is very much in line with the themes of My Little Pony.
The antagonist could be a rival chef. This chef might be jealous of Chowder and Mung's success and try to sabotage their restaurant or steal their recipes.
Well, a common theme is Chowder's attempts to overcome his own naivete. He's a young character, so fanfics might explore his growth in understanding the world around him. Also, the food aspect is often a big part of it. There could be fanfics where Chowder invents a new recipe that changes the whole culinary scene in Marzipan City.
Chowder fanfic lemon usually involves more mature and explicit themes. It might have intense emotional and romantic elements that are beyond the usual scope of the original Chowder content.
One plot idea could be a cooking competition where the main character has to make the best chowder and schnitzel. Another could be a story about a family that has a secret recipe for either chowder or schnitzel that they pass down through generations.
Adult fanfic is not something that should be promoted. Instead, you could look into Chowder's original concept art, the development of the characters in the official show, and how it influenced the animation industry in terms of style and storytelling.