I'm not sure who Aku specifically is. Aku could be a person with a great imagination who tells all kinds of stories, like adventure stories, fairy tales or even true - life stories.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
The character of Aku could be developed in a 'pregnant Aku in Samurai Jack fanfiction' by exploring how he interacts with the world around him differently. He may become more reclusive as he doesn't want others to see his 'weak' state. Or he could become more aggressive in trying to protect his pregnancy, which would be a new aspect of his character. Also, his relationship with his own power might change. Maybe he has to find new ways to use his magic that don't harm the unborn, which would force him to grow and adapt in ways he hasn't before.
Well,'sinopsis' might refer to a synopsis in Indonesian. 'Bahagia' means happy. 'Bukan' is 'not' and 'bidaan' could be something like a show or performance. So perhaps it means that a happy synopsis is not for a novel. But it's a bit unclear without more context.
One idea could be a crossover where Aku accidentally opens a portal to the Naruto universe. Naruto, being the curious and brave ninja he is, decides to explore Aku's world. They could start off as enemies, but gradually come to understand each other's motives and end up teaming up against a greater evil that threatens both of their worlds.
Since I'm not familiar with the 'Monster Dia and Meg story', it could potentially be a story about friendship between two monsters. Maybe Dia is big and strong while Meg is small and clever. They might go on a journey to save their monster homeland from some kind of threat, like an evil wizard or a natural disaster that is endangering their kind. However, this is all just speculation without more context.
Friendship is a very common theme. Since Chika and Dia have a strong bond, fanfics often explore different aspects of their friendship, like how they support each other through tough times.