Bonnie could be a great addition. She's Elena's best friend, and her magic could play a role in the fanfic. Maybe she helps Alaric and Elena when they are facing a powerful enemy. Also, her friendship with Elena can add depth to the story.
One great 'Alaric and Elena fanfiction' is 'A New Bond'. It delves into a different kind of friendship between them. Another one could be 'Alaric's Guidance for Elena' which shows Alaric as a mentor figure to Elena in a unique way. There's also 'The Unseen Moments' that focuses on their interactions that were not shown in the original series.
They would likely become closer. The shared experience of having a child on the way would create a strong bond between them. Alaric would probably be very protective of Elena.
Caroline would be a great character to include. As Elena's best friend, she can offer advice and support to Elena in her married life with Damon. Also, Stefan could be in the story. His relationship with Damon adds depth and complexity. He might be initially hesitant about their marriage but eventually come around to support them.
Perhaps a fanfic where Alaric, after his many experiences, decides to train Elena in some unique combat and survival skills. This could lead to a strong bond between them as they spend time together training and sharing their stories.
Sonic himself is a must - include. His speed and adventurous spirit drive the story forward. Tails is also great as he can provide technical support and is Sonic's loyal friend. And Dr. Eggman, as the main antagonist, always adds conflict and excitement to the story.
Vegeta is a great choice. His rivalry with Goku adds a lot of drama and action to a fanfic. Also, Piccolo, as he has a long history with Goku and has been both an enemy and a friend. Another good character could be Gohan, Goku's son, as it can explore their father - son relationship further in the fanfic.
Another good character to include is Lex Luthor. He could be the mastermind behind a plot to drive a wedge between Batman and Wonder Woman. His devious mind and resources would make him a formidable antagonist, and his interactions with the two heroes would add a lot of drama to the story.
Optimus Prime is a great choice. His leadership and wisdom can add depth to the story, and his relationship with Bumblebee can be explored further. Also, Sam Witwicky, as he has a strong bond with Bumblebee in the movies. Their friendship can be a central part of the fanfic.
Definitely Naruto himself, as he is the main character of the original series. Sasuke is also a great choice as his complex relationship with Naruto can add a lot of drama. Kakashi is another good character to include. His wisdom and unique abilities can be very useful in the story. And Sakura, her growth as a ninja can be explored further in a fanfic.
Well, obviously Armikasa themselves are the main focus. But you could also include their friends. For example, their comrades who are with them in training or on missions. These characters can add more depth to the story by providing different perspectives and interactions.