The baby could be a really interesting mix. Maybe it would have Finn's human features like his hair color and some of his facial structure, but also have the candy - like features of Princess Bubblegum. So it could have pinkish skin or some sort of candy - themed markings on its body.
The baby could have Finn's blue eyes and Princess Bubblegum's pink hair. It might also have a small crown - like feature on its head, just like Princess Bubblegum, but in a more baby - friendly version. And its clothing could be a mix of Finn's adventuring gear and the regal outfits of the Candy Kingdom.
Well, it could change the power dynamics. Finn being a human hero and Princess Bubblegum a powerful ruler, their baby might inherit unique abilities. It could also bring a new era of peace or new threats as different factions might react differently to this new addition. For example, the Ice King might see the baby as a new target for his schemes or as a potential way to gain some sort of power.
It could be a very interesting story. Maybe it would explore how Finn and Princess Bubblegum deal with the unexpected situation, like their reactions and how they plan for the future.
Jake, Finn's best friend, would probably be a very protective and fun 'uncle' to the baby. He might use his shape - shifting abilities to create interesting playthings or protect the baby from harm.
One possibility is that the story could focus on how Finn and Flame Princess adjust to their new roles as parents. They might face challenges in protecting their baby from various threats in the Land of Ooo.
The baby might have Bella's big, expressive eyes and Damon's dark, tousled hair. Maybe it would inherit Bella's pale complexion and Damon's sharp facial features. Since they both have a certain allure in their respective shows, the baby could be extremely beautiful or handsome, with an otherworldly charm.
The Ice King could be an antagonist. He might be jealous of their relationship and the new baby. He could try to kidnap the baby to have someone to care for him. Another potential antagonist could be some of the more traditional Candy People who don't approve of Marceline being with Princess Bubblegum and having a baby together. They might try to separate the family.
Their baby could look very much like a normal human baby at first glance, but on closer inspection, it has some mermaid - like qualities. For example, its ears might be slightly pointed like Ariel's, and when it's in the water, it seems to move with an unusual grace. It has a beautiful face with a combination of Ariel and Eric's best features, and its skin has a soft, almost iridescent sheen in certain lights.
One possible scenario is that she's trying to protect the Candy Kingdom from a new threat and Finn's impulsive actions get in the way. So, in her attempt to stop him from endangering the kingdom, she might accidentally hurt him. Another could be that she's under the influence of some sort of mind - controlling substance or magic and she ends up harming Finn without realizing it.
I think such a baby might have some fiery features, like red skin or flames around its body. But this is all just imagination.
One idea could be that the baby has unique powers that combine Princess Bubblegum's scientific knowledge and Marceline's vampiric abilities. For example, the baby might be able to control both candy elements and shadows. Another plot could involve the two moms trying to raise the baby in a world that doesn't fully accept their relationship, so they have to face challenges together to protect their family.