A big challenge could be trust. The humans in Will's world might not trust Bella because she's a mermaid and there are a lot of superstitions. And in the mermaid world, some mermaids might not trust Will because he's a human. Also, they could face environmental challenges. If there's a natural disaster affecting the ocean where Bella lives, Will might not be fully equipped to help due to his human limitations.
They could face challenges in trying to keep Bella's mermaid identity a secret. There could be nosy people in Will's town who suspect something strange about Bella. And for Bella, she might struggle to adapt to some of the human technologies and concepts that Will takes for granted. For instance, she might be confused by a simple thing like a cell phone at first.
One challenge could be communication. Since Bella is a mermaid and Will is a human, they might have different ways of expressing themselves. For example, mermaid language and signs might be difficult for Will to understand at first. Another challenge could be dealing with the different laws and rules of their two worlds. Mermaid society might have rules about interacting with humans that Bella has to navigate while Will might face problems in his human world if he is seen spending too much time with a'mysterious girl' like Bella.
A possible plot could be that they become friends and start a project to educate humans about mermaids in a non - romantic way. They could work together to create a school program or something similar. For example, Will could bring in his schoolmates and Bella could show them some of her mermaid abilities in a safe and educational setting.
Maybe the kiss is part of a spell or a curse. In the world of H2O, there could be some magical force at play. The kiss was meant to transfer powers or memories between them. So, instead of a normal romantic kiss, it becomes a catalyst for a whole new adventure or problem they have to face together.
One possible plot twist could be that the pregnancy is not as straightforward as it seems. Maybe there are some magical or supernatural elements involved in Bella's pregnancy that Will has to figure out. For example, it could be that an ancient spell has been cast on Bella which causes her pregnancy, and this spell has some hidden intentions or consequences that will unfold throughout the fanfiction.
One common theme is friendship turned into love. Since Bella and Will are friends in the h2o universe, many fanfictions explore how that friendship can gradually develop into something more romantic. Another theme could be adventure. They might go on some underwater adventures together that strengthen their bond.
One common theme is friendship. Since they are both part of the h2o world, their friendship is often explored. They might support each other through tough times, like when dealing with other characters who don't understand their mermaid/merman nature.
One common theme is friendship. Fans often explore how Will and Bella's friendship develops over time, especially considering Bella's mermaid secret. It's about their loyalty and how they support each other.
One challenge could be the reactions of their families. If their families didn't approve of their relationship, the pregnancy might cause more tension. Another challenge could be financial. Preparing for a baby is expensive, and they might struggle to afford all the necessary things like diapers, clothes, and a crib. Also, Bella's health during the pregnancy could be a concern. There could be complications that require special care.
I'm not sure exactly which fanfiction you're referring to specifically. There could be many different stories out there about Bella and Will from 'H2O: Just Add Water'. You might want to check fanfiction websites like FanFiction.net or Archive of Our Own to find the one you're interested in.
In the fanfiction, Carlisle's sense of duty to his family and the vampire code might be a hurdle. He has to balance his feelings for Bella with his responsibilities to the Cullen family. Also, Bella's own human life and her relationships with other humans, like her father, could be a complication. How can she maintain those relationships while being in love with Carlisle? There's also the issue of secrecy. Their relationship has to be kept hidden from the outside world to protect both of them.
Another challenge is the physical and emotional transformation Bella goes through. When she decides to become a vampire to be with Edward forever, she has to endure the pain of the transformation. And emotionally, she has to give up her normal human life, like going to college in a traditional way, and deal with the changes in her relationships with her human loved ones. It's a complex and difficult process for both of them.