The main antagonist could be the current ruler who has unjustly taken the crown from the rightful king. This character would be power - hungry and do whatever it takes to keep the throne, including plotting against the king without a crown.
You might find various storylines. Maybe it'll focus on the king's struggle to regain his crown or his adventures in a new land without his former status.
In a 'a king without a crown fanfic', the king might be cursed to never wear the crown again. So, he focuses on building relationships with neighboring kingdoms on a personal level rather than through diplomatic formalities. His charm and wisdom begin to earn him respect from other rulers, and he starts to form alliances that could potentially help him break the curse and reclaim his crown.
A powerful rogue Shinigami could be the main antagonist. This Shinigami might be jealous of the Shinigami King's power and wants to overthrow him. He sees Naruto as a threat because of his connection to the king and tries to stop Naruto from fulfilling any tasks or missions related to the king. He could also be causing chaos in the living world by releasing dangerous spirits or curses.
A powerful organization that wants to use the power of the Fox King for world domination could be the main antagonist. They see Naruto as an obstacle because of his connection to the Fox King. JM, being associated with Naruto, also becomes a target. This organization might have their own set of powerful ninjas and dark jutsu, and they stop at nothing to achieve their goal of harnessing the Fox King's power.
To draw a cartoon king without a crown, begin with a rough draft of the king's stance. Detail the hair and ears. For the clothing, imagine unique patterns but without a crown on top. Add some colors to make it more vivid.
Powerlessness is a common theme. Without his crown, the king has lost his symbol of authority and power, so stories often explore how he copes with this new state of being powerless. Another theme is redemption. He may be on a path to regain his crown and in the process, redeem himself for whatever led to him losing it in the first place. Identity crisis is also prevalent as he is no longer the king in the traditional sense and has to redefine who he is.
Perhaps a group of anti - technology extremists could be the main antagonists in an Ultron fanfic. They see Ultron as the symbol of all that is wrong with technology and are determined to take him down, no matter the cost. They might use their own advanced, but more rudimentary, technology to fight against Ultron. This would create an interesting contrast between Ultron's high - tech abilities and their more basic, yet effective, methods.
Maybe it's because such a concept adds an element of mystery and drama. People are drawn to the idea of a powerful figure facing unusual circumstances.
Madara Uchiha could also be a great antagonist. Imagine if he came back and thought that the power of the Soul King could be used to achieve his ultimate goal of creating a world under his control. He'd be a formidable foe for Naruto, especially since he has so much experience and power already.
Perhaps an organization similar to Akatsuki. They might see the king of beasts as a powerful tool to achieve their goals, whether it's collecting all the tailed beasts or just ruling the world with an iron fist. They would send their strongest members to try and capture the king of beasts, and Naruto would have to stop them. In this fanfiction, this antagonist organization could have new and more powerful abilities compared to the original Akatsuki in the Naruto series.