It could be a dark version of Christopher Robin himself from an alternate dimension. This 'evil' Christopher Robin might be jealous of the happiness in the real Hundred Acre Wood and try to disrupt it. Pooh would have to rely on his pure heart and the love he has for his friend to bring the real Christopher Robin back and defeat the impostor.
One idea could be that Christopher Robin gets lost in the Hundred Acre Wood as an adult and Pooh has to help him remember all the wonderful things from his childhood. They could go on a journey to find the special places that they used to visit together, like the bridge over the stream or the bee tree.
The significance was that he was the real - life model for the famous character. His existence and his relationship with his father led to the creation of the iconic Christopher Robin in the stories.
One interesting 'Christopher Robin fanfic' could be about Christopher Robin's adventures in a modern world. He might discover new technologies and how they compare to the simple joys of the Hundred Acre Wood.
You can try fanfiction.net. It has a wide range of fanfics from various fandoms, and there's a good chance you'll find some great 'Christopher Robin fanfic' there.
Christopher Robin is based on the childhood friendship of author A. A. Milne's son with his stuffed toys, particularly Winnie-the-Pooh. It explores how that childhood imagination influences his adult life.
The story about Christopher Robin is not based on real events. It's part of the imaginative world of literature and entertainment. It was crafted by an author's creativity rather than being based on actual happenings.