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.
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.
I'm not sure who specifically wrote the lyrics for a song titled 'lirik never ending story'. It could be written by the songwriter associated with a particular musical version of 'The Never Ending Story'.
It's possible that Ada and Claire are part of a fantasy world. Ada could be a shape - shifter or a magical being, and Claire could be a human who has stumbled into Ada's world. The licking of the neck could be a sign of recognition, like Ada is identifying Claire as someone special or as a potential threat in that world.
Ada and Helena are likely characters from a particular fictional universe, perhaps from a game or a book series that has a fan base creating fanfiction about them.