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.
Perhaps it's a dark entity that has invaded the Hundred Acre Wood. This entity could be jealous of Christopher Robin's happiness and decides to turn his world into a horror. It targets him by using his friends and the familiar places against him. So, this unknown dark entity might be the antagonist.
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.
How about Ra's al Ghul? He has his own ideas about justice and order that are very different from Batman's. He could see Robin as a potential recruit for his League of Shadows. Ra's al Ghul could manipulate events to make Robin question Batman's methods and try to draw Robin to his side.
A jealous carnival owner could be the antagonist. He doesn't like Beppi teaming up with Djimmi because he fears their combined popularity will overshadow his own carnival. So he tries to sabotage their shows and separate them.
It could be a rogue ninja who has a grudge against their village. This ninja might target Chocho and Karui because they are seen as rising stars in the village. The rogue ninja could try to disrupt their training or missions, causing trouble for them and testing their friendship.
A jealous gamer who doesn't like the idea of Vanellope and Hiro teaming up could be the antagonist. This person might try to hack into the systems that connect their two worlds and cause chaos.
It could be Orochimaru. He's always been interested in unique powers and abilities. The idea of X - 23's mutant abilities and Naruto's jinchuriki powers would be very appealing to him. He might try to capture them both to use in his experiments to gain ultimate power.
A new, self - made villain who is obsessed with colors and wants to collect all the 'powerful' colors in the world. Robin's red stands out to him as one of the most important ones to obtain, so he sets out to defeat Robin and take his red essence.
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.
In a different take, a new character could be introduced as the main antagonist. Maybe a criminal mastermind who teams up with the Joker specifically to target Robin. This new character could have a personal vendetta against Robin or see him as a key to achieving some greater evil plan, and the Joker is just a means to an end in this scenario.