Maybe it has a more reserved personality like Branch at first. A bit shy but with a big heart. As it grows, it could start to come out of its shell more, especially with the help of its parents and the friendly Trolls around. It could be really creative, using things from the forest to make little artworks.
Perhaps the story is about the baby being a symbol of hope for the future of the Trolls. Poppy and Branch would be protective parents. They would teach the baby about love and harmony. There could be a plot where an outside threat tries to harm the baby, but the whole Trolls community unites to protect it.
The baby could inherit Soul's coolness. Maybe it would be a laid - back baby who is not easily flustered. Or it could have Maka's determination. So when it wants something, like a toy, it would be very persistent in getting it.
The baby might be very inquisitive. With Athena as its mother, it would inherit a thirst for knowledge. It could constantly be asking questions, eager to learn about the world around it, whether it's the secrets of the ocean (from Percy's side) or the wisdom of the heavens.
The baby could be really feisty. Since Sesshomaru is a powerful demon, the baby might inherit some of that strength and show it early on as a spunky and fearless personality.
Another possibility is that the baby has a combined power. It could use water to create illusions, a mix of Percy's water control and Annabeth's strategic thinking. So when in a fight, it could create false images of itself using water to confuse the enemy and then attack from an unexpected angle.
The baby could potentially have Naruto's chakra powers. Since Naruto has a vast amount of chakra and can access the Nine - Tails' power, the baby might inherit some of that. Maybe it could also have some unique abilities from the OC. For example, if the OC has a special type of jutsu related to nature manipulation, the baby could have a combined form of that and Naruto's powers.
They might have a baby that has a combination of their physical features. For example, it could have Foxy's red fur and Mangle's long limbs. Maybe it also has Foxy's eye patch on one eye and Mangle's multiple tails.
The baby could inherit Naruto's chakra reserves and his ability to use the Rasengan. Since Naruto has an enormous amount of chakra, it would make sense for the baby to also have a large chakra pool. The Rasengan is one of Naruto's signature moves, and it could be passed down genetically or through training that Naruto would give the baby as it grows up.
There's a possibility that the baby could inherit Knuckles' connection to the Master Emerald. Maybe the baby would have some sort of enhanced intuition when it comes to protecting powerful artifacts. From Rouge, the baby could inherit her stealth skills. Rouge is very good at sneaking around, and this could be a useful ability for the baby to have, especially if they get into adventures in the fanfiction world.
The baby could have Naruto's ability to use the Rasengan. Since Naruto has mastered this powerful technique, it might be passed down genetically or through training. Also, it could inherit Samui's intelligence and strategic thinking. This combination would make the baby a very formidable ninja.