Well, surely Princess Cadence herself is a main character, even though she might die in the story. Her husband Shining Armor would likely be a main character as well, as he has a very close relationship with her. Twilight Sparkle, being Cadence's sister - in - law, would also probably be a major part of the story, since she would be deeply affected by Cadence's death.
It could be that this fanfic is a rather dark exploration of the My Little Pony world. It might show the breakdown of relationships after Princess Cadence's death. For example, friendships between ponies could be strained as they deal with their grief in different ways. It could also touch on themes of hope and recovery, like how the ponies try to move forward and restore the kingdom to its former glory without her.
Shining Armor, her husband, would surely be a main character. He could be shown as a very supportive and protective partner. He might be by her side during doctor visits, making sure she is comfortable and taking care of all her needs.
Princess Bluebelle is likely one of the main characters. And probably some of the well - known My Little Pony characters like Twilight Sparkle, Pinkie Pie etc. might also be involved depending on the plot.
Often, these types of fanfics can be a great way to expand on the characters. They might show a different side of Shining Armor, perhaps a more vulnerable side when it comes to protecting his family. And Cadence could be shown as not just a princess of love, but also a strong leader. The fanfic could also explore their friendships with other ponies in Equestria.
A good fanfic about them should have a strong plot. It could be about them solving a mystery in Canterlot. Also, the characters need to be in - character. Shining Armor should be brave and loyal, and Cadence should be kind and full of love.
Typically, you might find Twilight Sparkle among the main characters in 'cupcakes mlp fanfic'. Since she is a curious and intelligent pony, she could be involved in the cupcake - related events, perhaps studying the magic behind making the perfect cupcake. And of course, Fluttershy might also be there. She could be contributing her gentle touch and love for animals to the cupcake story, like using honey from her animal friends for the cupcakes.
Twilight Sparkle would definitely be one of the main characters affected. As Spike was her assistant and friend, his death would leave a huge void in her life.
I don't know for sure as I haven't read it. But it could be Twilight Sparkle and her friends like Pinkie Pie, Rainbow Dash, etc. Since they are the main characters in the My Little Pony series, they are likely to be involved in any fanfic related to it.
Typically, the main characters would include the Power Ponies themselves. These are ponies with special powers. There might also be some recurring villains that play a major role in driving the story forward.
I'm not sure exactly who the main characters are without reading it. But it could be Twilight Sparkle as she is often involved in magical adventures. Maybe she discovers the time turner and is the central character in the story.