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, a plot could revolve around Aku trying to keep his pregnancy a secret from Samurai Jack. He might be worried that Jack would use this new situation to his advantage. And then there could be a sub - plot about Aku having strange cravings that lead him to unexpected places in the world of the fanfiction, causing chaos in different ways.
One common theme is the struggle between good and evil, as Jack represents good and Aku represents evil. Another is the concept of fate, as their battles seem pre - ordained.
One great Samurai Jack fanfiction story is 'The Lost Years of Jack'. It delves into what Jack might have been through during his time away from his home. It's really interesting as it shows his struggles and how he managed to keep his spirit alive in the face of so many challenges.
It depends. Some consider them canon if they fit seamlessly into the established Samurai Jack universe and don't contradict the main storylines. Others might have different opinions.