Perhaps they could form an unlikely friendship. Rumble, being a pegasus colt, and Sweetballe, with her own unique charm, might start off not getting along but then discover they have more in common as they work together to help a group of young ponies learn about flying and friendship. They could then have to defend their newfound friendship from those who don't understand it.
Well, clearly Sweetballe and Rumble are the main characters. Sweetballe might be a pony with a kind heart and some special abilities or qualities. Rumble, as a pegasus, is likely to have flying skills and a personality that can either complement or contrast with Sweetballe's.
Perhaps a story where Scootaloo and Rumble get into an argument over which pony tribe is better, the pegasi or the earth ponies. But then they are forced to work together when a strange magic starts affecting the ponies of their respective tribes. As they work together, they realize that each tribe has its own strengths and that they were wrong to argue in the first place.
One idea could be a pony being reborn with memories of a past life in a different pony world. They could use that knowledge to bring about positive changes in Equestria, like finding new ways to solve friendship problems.
One idea could be that the Dazzlings try to reform and join the main group of ponies in Ponyville. They struggle to overcome their past of using dark magic to steal harmony and learn to use their powers for good.
Well, a plot could involve a villain trying to steal all the cupcake recipes in Ponyville. The ponies then have to work together to protect their precious recipes. It could also be about a pony who has lost their sense of taste and has to go on a journey to regain it by trying different cupcakes. Or perhaps there's a mystery surrounding an ancient cupcake recipe that holds a great secret.
One interesting plot could be the human adjusting to the magical world of Equestria. For example, learning to use magic or getting used to the different species and their cultures. Another idea is the ponies trying to understand human behavior and needs. They might be confused by things like human technology or the lack of a horn or wings.
One interesting plot idea could be a mystery during the wedding. For example, a valuable item goes missing right before the big moment. Another could be a character from the guests' past shows up unexpectedly and has some unresolved issues with the bride or groom. A third idea might be that there are some magical mix - ups that cause comical chaos during the ceremony.
A really interesting plot could be a pony from the other side who has unique powers that are considered dangerous in their own world. They flee to the normal pony world and have to hide their powers while also trying to find a place where they belong. This could involve a lot of character development as they interact with the normal ponies and learn about a different way of life.
One idea could be that Twilight Sparkle accidentally activates a time - travel spell and ends up in the past, right when Equestria was first being founded. She has to figure out how to get back while also making sure she doesn't change history too much.
One idea could be a time - travel plot where the Equestria Girls go back in time to the founding of Canterlot High. They could witness the first magic being used there and have to ensure that history isn't changed too much. Another idea is a crossover with another magical school from a different franchise. For example, they meet students from Hogwarts and have to learn from each other's magic systems. And a third idea could be a story where they lose their magic and have to go on a quest to find a way to get it back, encountering many challenges and new friends along the way.