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 is often popular. Her intelligence and leadership skills in the original series make her a great candidate for reborn fanfiction. Writers can explore how she might use her past experiences in a new life to further her magical studies or help Equestria in different ways.
Twilight Sparkle is often very popular in fusion fanfiction. Her powerful magic makes her a prime candidate for fusions. When fused with other characters, she can bring a lot of intelligence and magical ability to the new entity. Rainbow Dash is also popular. Her speed and athleticism can add an exciting dynamic to any fusion she's part of.
Twinkle Twirl is often a beloved character. Her kind and gentle nature makes her a favorite in many fanfictions. Another one is Firefly. With her adventurous spirit, she gets into all sorts of exciting situations in the stories. And of course, there's also Cotton Candy, who is adored for her sweet personality.