Perhaps there could be a story where Thomas and Edward compete in a friendly race. Along the way, they both learn important lessons about sportsmanship and friendship. For example, when one of them has a minor accident, the other stops to help, realizing that winning isn't everything.
In fanfiction, they could face a big crisis together, like a major bridge collapse that blocks the railway. As they work side by side to fix the problem and keep the railway running, their bond of friendship would grow stronger. They might have to rely on each other's unique skills, with Thomas being fast and Edward being strong.
There might be a fanfiction where Edward has to deal with a strange malfunction that gives him unique abilities. For example, he could suddenly be able to understand the thoughts of all the other engines. This would lead to some very interesting interactions and situations as he tries to keep this a secret while also using it to help his friends.
One interesting story could be about Edward and Emily working together to rescue a stranded train. They use their combined strength and smarts to get the job done.
James is often prideful and Edward is more kind - hearted. In a fanfiction, their different reactions to praise can show this. When the Fat Controller compliments them, James might puff up with pride while Edward just smiles modestly.
One plot idea could be that James and Edward get into a friendly competition to see who can pull the most cars in a day. Another could be that they have to work together to rescue a stranded train during a storm. Maybe there's a situation where James gets in trouble for being too boastful and Edward has to help him learn a lesson about humility.
One great fanfiction could be about Thomas and Percy's adventure in a new part of the island. Maybe they discover a hidden track that leads to a magical place full of talking animals and beautiful scenery. They have to work together to get back home and tell the other engines all about it.
One common theme is friendship. Since they are both engines in the same railway universe, stories often explore how they support and care for each other. For example, Thomas might get into trouble and Edward comes to his rescue. Another theme is adventure. They could go on exciting trips together or explore new parts of the railway network.