It can even be a basic Slice of life story, like if Alfred never sent Lazuli and Leaf away, where thanks to his daughters intervention, he dosen't kill the criminals and instead has to raise them as a single father, with all of them grieving over their lost mother/wife. Heck, you can even do it with original characters. Take my More Family story, for example. In it, Speed is not the main character. Instead, I give him an older sister named Rose, who is the main character. She has her own personality, goals, and story. She also isn't a copy-paste of Speed or any characters. She is responsible, like Speed, kind, like Flare, and fun, like Crystal. However, OC stories are probably not recommended since you need to make sure they are not too out of place. Making a over powered character, or an extremely popular character, or basically a character with no negative traits, isn't fun to read about, and is kind of hard to make. So, to answer your question, it all really depends on the what if you want to make. If it is about Speed, I don't know, being part Werewolf, then Speed should be the main character. If it is about Black not taking the S rank mission, then it Black should be the main character. If it is about maybe Lazuli has a uncle who takes them in after Alfred's arrest, them have it be about either Lazuli, Leaf, the Uncle, or all 3. It is possible to have more them one protagonist, like in my Blue and Yellow Adventures story, where both Speed and Lazuli are the main characters. So, it is up to you.
