Tempest Shadow is also a great character to include. Her story of being broken and then finding redemption can be further explored. Maybe in the fanfiction she has a relapse of her insecurities and has to fight to overcome them again.
The Storm King himself is very interesting. He has a menacing presence and his motives for conquering are open to exploration. You could delve into his past to find out what made him so power - hungry.
The ponies, especially Twilight Sparkle, are always interesting. Twilight could be in a situation where her magic is not enough to defeat the Storm King at first. This would force her to think outside the box and use her intelligence and the power of friendship in new and creative ways.
Definitely Pikachu from Pokemon. Pikachu is super cute and its electric powers would be really cool to see in Equestria. And from My Little Pony, Twilight Sparkle would be great. She's really smart and into magic, so she could be very interested in learning about Pokemon. They could have some really fun interactions together, like Pikachu using its electricity to light up Twilight's library when there's a power outage or something.
Twilight Sparkle's human counterpart is often really interesting. She usually retains her intelligence and love for knowledge, which makes her stand out. Then there's Rainbow Dash's human form. With her athletic and competitive nature, she can be really engaging in the human fanfiction world.
A fanfiction could focus on how the ponies form an unlikely alliance with some of the Storm King's minions. They realize that they also don't like the Storm King's tyrannical ways and decide to work together to overthrow him. This would involve a lot of character development for both the ponies and the minions.
In many fanfictions, there's a Bat Pony leader character. This character is usually strong and wise, guiding the Bat Pony community. For example, a character like Shadowwing. Also, there are Bat Pony characters who are hybrids, part Bat Pony and part other magical creature. They tend to be popular as they have unique abilities and backstories. They can be both a source of intrigue and a bridge between the Bat Pony world and the rest of Equestria.
Celestia is often paired with King Sombra. Their power levels are somewhat similar, and they represent opposite sides of good and evil, so it makes for an interesting dynamic. Twilight Sparkle is also a common character to be paired with him. Given Twilight's knowledge of magic, she could either be in a battle of wits with Sombra or trying to understand his magic for some reason.
Twilight Sparkle is often really interesting in fanfictions. Her powerful magic and her thirst for knowledge make her a great character to build stories around. Rainbow Dash is also cool, with her speed and her daredevil attitude.
Ben and Twilight Sparkle would be really interesting. Ben has his street - smart and action - oriented nature, while Twilight is all about knowledge and magic. Their interactions could be really fun, like Ben showing Twilight some of his alien tech and Twilight trying to understand it with her magical knowledge.
Twilight Sparkle and Naruto are often very popular. Twilight's thirst for knowledge can be a great parallel to Naruto's determination to become stronger. Also, Rainbow Dash with her athleticism can be compared to some of the more physically adept Naruto characters.
Twilight Sparkle is a top pick. Fans like to explore how her logical mind might be warped by hypnosis. Rarity could be popular as well. Her sense of style and vanity might be distorted in hypnotic situations. And Applejack, with her strong will and sense of responsibility, can be an interesting character to write about when hypnotized as readers can see how that will is tested.
Twilight Sparkle is often very popular. Since she's the studious and magic - oriented pony, her reactions to new magical elements or events are always interesting to explore. Rainbow Dash is also popular. Her confident and sometimes brash nature means her reactions to challenges or competitions can be really exciting to read about.