It could be Bowser. He might see the new baby as a way to get to Daisy and Luigi. He could try to kidnap the baby to use as leverage against them.
Maybe some jealous characters in their world. They could be envious of Daisy and Luigi's new family and try to create problems for them. For instance, they might spread false rumors about the baby or try to cause trouble between Daisy and Luigi in the chaos of new parenthood.
There could be an ancient evil force that is awakened by the birth of the baby. This force sees the baby as a threat or as a key to achieving its own dark goals. So it sets out to target the baby and Daisy and Luigi have to protect their little one from this mysterious and powerful foe.
Peach could be an important character. As a friend of Daisy and a princess in the Mushroom Kingdom, she could offer her wisdom on raising a child. Also, Yoshi could be there. Yoshi is always friendly and could be like a protector for the baby, watching over it when Luigi and Daisy are busy.
Well, perhaps the plot could involve Daisy and Luigi facing the challenges of new parenthood. They might struggle to balance their own lives and duties while taking care of the baby. For example, Luigi could be worried about protecting the baby from the various threats in their world, while Daisy is focused on the baby's growth and development.
One possible plot could be that Luigi and Daisy discover they are going to have a baby. They then start preparing their home, like painting a nursery. Luigi might be a bit nervous but also excited, and Daisy could be the strong one, reassuring Luigi throughout the process.
Adventure is also common. They could be on a quest to find a lost item or to explore a new part of their world. It makes for exciting reading as they face various challenges as babies. For instance, they might encounter a small stream that they need to cross but are too small to do so easily. And they have to figure out a solution together. This kind of adventure - based theme is quite popular.
A really fun 'baby daisy and baby luigi fanfiction' could be set in a world of toys. They are toys themselves and they have to work together to escape from the clutches of an evil toy collector. They make friends with other toys along the way and use their unique skills, like baby Daisy's ability to find hidden paths and baby Luigi's courage, to overcome the challenges.
Wario could also serve as an antagonist. He might be envious of Luigi's new married life and try to steal their treasures or disrupt their happiness. Maybe he spreads false rumors about Luigi in the Mushroom Kingdom just to create chaos in their lives.
You can start by focusing on their innocent personalities. For example, show baby Luigi sharing his favorite toy with baby Daisy. This simple act can create a warm feeling in the story.
One popular 'baby Luigi and baby Daisy fanfiction' could be those that focus on their adventures in the Mushroom Kingdom as babies. For example, there might be stories where they discover new power - ups together in a cute and innocent way.
Mario would definitely be a main character. Since Luigi is his brother, he would be the one most concerned about Luigi's transformation and would do everything he could to help him. Princess Peach could also be a main character. She might offer her castle as a safe place for baby Luigi and use her resources to find a solution. Toad could be there too, helping Mario with taking care of Luigi and providing useful information about the kingdom that could lead to reversing the spell.
In this fanfiction, there might be some werewolves who view the baby as an abomination since it's part vampire. They could try to harm the baby, which would put Bella and Stefan in a difficult position to defend their child.