Yes, 'Over the Garden Wall' is based on a classic novel. The story draws inspiration from various classic literary elements and themes, which gives it a rich and timeless feel.
Another interesting angle is to focus on the side characters. You could write a fanfiction about the Woodsman's past. What was his life like before he got trapped in the never - ending task of keeping the lantern lit? It could be a tragic love story or something related to his family.
Begin with creating an outline. Decide on the starting point, the middle events, and how you want it to end. If you're writing about a new adventure for the brothers, list out the different obstacles they'll face. For example, new strange creatures or mysterious places. And don't be afraid to add your own unique twists to the story.
There are many interesting fanfictions that combine these three universes. Some might focus on characters from each series meeting in a crossover adventure. For example, Harry Potter characters could find themselves in the strange and magical world of Gravity Falls, or in the eerie yet enchanting world of Over the Garden Wall. The possibilities are endless for exploring different character dynamics and plotlines.
First, consider the aesthetic of the show. The OC should fit in with the dark, autumnal, and slightly surreal world of the Unknown. For example, give them clothing in earthy tones and perhaps a tattered cloak. Second, think about their backstory. Maybe they were once a normal person from a nearby village that got caught in the strange magic of the Unknown. Third, give them a unique ability or trait that is relevant to the world, like being able to communicate with the crows that are often seen in the show.
One idea could be an OC who is a lost spirit in the Unknown. They have the power to manipulate time briefly but don't fully understand it. They cross paths with Wirt and Greg and end up joining them on their journey, causing some unexpected time - loop situations along the way.
Another interesting angle is to write from the perspective of one of the side characters in the show. For instance, the Woodsman. You could delve into his backstory, how he came to be in the service of the Beast, and his internal struggle between his love for his daughter and his fear of the Beast.