Often as a very close friendship. They're shown supporting each other. For example, Rainbow Dash might encourage Pinkie Pie when she's feeling down about a party not going well. And Pinkie Pie would cheer on Rainbow Dash during a big race.
One popular one could be 'The Adventures of Rainbow Dash and Pinkie Pie'. It often has them going on wacky adventures together in Equestria, like exploring a mysterious new part of the Everfree Forest. They face challenges and their different personalities, Rainbow Dash's boldness and Pinkie Pie's randomness, make for really interesting interactions.
Character development is key. In these fanfics, we often see Rainbow Dash becoming more in - touch with her softer side thanks to Pinkie Pie, and Pinkie Pie might become more responsible with Rainbow Dash's influence. Also, the fanfic should capture their unique personalities well. Rainbow Dash's competitiveness and Pinkie Pie's wackiness should be on full display.
There are quite a few 'rainbow dash x pinkie pie fanfic' out there. Some focus on how they support each other during tough times in Ponyville. Like when there was a big storm and Pinkie Pie used her cheerfulness to lift Rainbow Dash's spirits, and Rainbow Dash in turn protected Pinkie Pie from the strong winds.
A popular fanfic in this pairing is 'The Bond Between Colors'. In this, their friendship blossoms into something more special. It delves into how they face challenges together in Ponyville. For example, when there's a problem with the local fashion show and Rainbow Dash uses her speed to help Rarity get all the materials she needs in time. It's a really heartwarming read.
A good fanfic of them should have their distinct personalities shine through. Rainbow Dash's confidence and athleticism, and Pinkie Pie's wackiness and friendliness. If it can capture that essence, it's a great start.
In a Rainbow Dash and Pinkie Pie fanfic, it could be centered around their shared experiences. Maybe they enter a competition together. Since Rainbow Dash is competitive and Pinkie Pie loves to be involved in things, this could lead to some really interesting scenarios. The story might also explore how they deal with their differences. For example, Rainbow Dash might get frustrated with Pinkie Pie's chaos at times, but then realize that it's part of what makes her special. And of course, there will likely be lots of colorful descriptions of Equestria and the other ponies they interact with.
Friendship is a very common theme. Since they are both characters from the MLP universe, fanfics often explore how their friendship develops and grows. They might start as acquaintances and become best friends, or they might have some bumps in their friendship that they need to overcome.
Adventure is also a popular theme. They might go on a quest together, like looking for a lost treasure or saving Ponyville from a mysterious threat. This allows for a lot of action and character development. In these types of fanfics, Rainbow Dash's bravery and Pinkie Pie's spontaneity often play important roles.
Perhaps the OC has a special talent related to baking, like creating unique and magical pastries. Pinkie Pie discovers this and they team up to create the most amazing party treats ever. They could go on adventures together to find rare ingredients and along the way, form a strong friendship.