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.
To open the National Karachi app, one first had to click on the National Karachi icon on the phone's desktop. Then, he went to the National K Songs homepage and clicked on the K Songs option. Next, choose the song you want to sing on the song selection interface. After entering the interface of the song, click on the button I want to karaoke to start using the National karaoke song. If you wanted to use the app on a computer, you could download the app or use the web version to open the app.
The computer version of National Karachi did not have a karaoke room. If one wanted to enter the karaoke room, they could only use the National Karakorum APP on their phones. On the home page of the APP on the phone, you could find the karaoke room function and click to enter. If no one invited them, they could only enter the karaoke room without a password. In addition, VIP users could gift karaoke rooms to non-VIP users so that they could create their own karaoke rooms. If you want to enter the karaoke room on your computer, you can install an Android compatible simulator on your computer, such as BlueStacks or NoxPlayer, and download and install the PC version of the karaoke app in the simulator.
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, not knowing the exact details, it might have a complex love story. Maybe there are characters who face various challenges in their relationships, as is often the case in many novels.
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.