A plot could involve a villain that is causing chaos in both Sonic's world and Equestria. Sonic and Rainbow Dash team up to stop this villain. They would have to combine their unique skills. Sonic's speed and ability to spin dash could be used in combat, while Rainbow Dash's flying and weather - controlling powers (like creating strong winds to blow enemies away) would also play a crucial role. Along the way, they learn about each other's worlds and make new friends in the process.
One idea could be that Sonic accidentally enters Equestria where Rainbow Dash lives. They could then have a race to see who is faster in the new and unfamiliar terrain for Sonic. Rainbow Dash might be amazed by Sonic's speed and Sonic could be intrigued by Rainbow Dash's ability to fly while racing.
Sonic would probably be really excited to meet Rainbow Dash. He might challenge her to a friendly race right away. Rainbow Dash, being competitive herself, would accept without hesitation.
Perhaps a story where Dark Sonic is being chased by some dark forces and he hides in Rainbow Dash's cloud home. Rainbow Dash discovers him and at first is scared but then decides to help him. Together, they fight off the dark forces, with Rainbow Dash teaching Dark Sonic about friendship and loyalty along the way.
A possible plot is that Scootaloo discovers a hidden talent for speed flying but is too shy to show it. Rainbow Dash notices and encourages her to enter a special speed - flying event. Also, they could be on a quest to find a lost Pegasus herd and along the way, Rainbow Dash imparts her wisdom about friendship and loyalty to Scootaloo. There could also be a story where Scootaloo accidentally breaks Rainbow Dash's favorite item and tries to make it up to her in a really heartwarming way.
One idea could be Sonic crossing over with another popular game like Mario. Sonic could find himself in the Mushroom Kingdom and have to team up with Mario to stop a new threat that combines the chaos emeralds and power - ups from both worlds. Maybe Bowser has teamed up with Dr. Eggman to create this chaos.
One idea could be Rainbow Dash joining the Navy OCS (Officer Candidate School) in the MLP universe. She could face challenges like learning complex flight maneuvers specific to naval operations. Maybe she has to work with other ponies she doesn't get along with at first, but they grow to respect each other through their training.
One idea could be that Rainbow Dash and Soarin enter a flying competition where the prize is a rare lemon - scented flying charm. Along the way, they face challenges that test their friendship and flying skills.
One idea could be that Sonic accidentally enters the world of Equestria Girls while chasing Dr. Eggman through a strange portal. He then teams up with the girls to stop Eggman from stealing the magic that powers their world. Another could be a race between Sonic and Rainbow Dash which gets interrupted by a common enemy.
Another theme is competition. Given their speed - based abilities, it's not uncommon for fanfics to pit Rainbow Dash and Sonic against each other in races or challenges. However, these competitions usually end up with them learning from each other and becoming better friends in the process. It's a way to explore their personalities and how they deal with winning and losing.
Overcoming differences is another theme. Sonic is from a high - tech, fast - paced video game world, while Rainbow Dash is from a magical pony world. In the fanfic, they have to overcome these differences in their cultures, abilities, and ways of thinking to work together effectively.
One idea could be that Sombra, who has reformed, teams up with Rainbow Dash to compete in a special Equestria - wide flying and magic - using tournament. They start off as reluctant partners but gradually learn to respect and rely on each other's skills.