One idea could be that the chipmunks get caught in the middle of a territorial war between two groups of forest animals. Maybe they have to use their musical talents to broker peace.
One idea could be an OC who is a talented musician but is shy. The Chipmunks discover this OC and help bring out their confidence on stage. For example, they could encourage the OC to perform at a local talent show.
One plot idea could be that the sister chipmunk joins the boys on a musical tour and has to overcome stage fright. Another could be a story where she discovers a hidden talent for songwriting that helps the group create a new hit album. Maybe she gets lost in a big city during a promotional event and has an adventure while the boys are searching for her.
One idea could be to write a fanfic where the chipmunks go on a magical adventure in a new, enchanted forest. They could discover new friends and face challenges together.
One idea could be a crossover with the Smurfs. Alvin and the Chipmunks could accidentally wander into the Smurf village while on an adventure in the forest. They might initially be seen as strange little creatures by the Smurfs, but then they team up to save the village from Gargamel who has a new plan that involves using the Chipmunks' musical talents to lure the Smurfs.
Maybe there's a competition in their town, like a battle of the bands. The Chipmunks and Chipettes have to work together to create a unique performance that showcases their individual talents as well as their ability to collaborate. They face challenges along the way, like a broken instrument or a member getting sick, but they overcome them with their friendship and determination.
One idea could be a story where the Chipmunks become human for a day and experience high school. They could struggle with typical teenage problems like fitting in and dealing with bullies.
Well, there are fanfictions where the Chipmunks meet famous human musicians. For example, they could have an encounter with Elvis Presley. In this story, Elvis is so impressed by their unique voices that he wants to collaborate with them. Or perhaps a fanfiction where Alvin gets lost in a big city and has to rely on his wits and the help of some friendly pigeons to find his way back to his brothers. And then there's the one where they use their music to save a small town from a group of bullies who are trying to take over the local music scene.
A great Alvin smart fanfic might be where Alvin figures out a way to save the Chipmunks' tree house from being demolished. He uses his intelligence to find a legal loophole or comes up with a brilliant plan to raise enough money to buy the land it's on. For example, he might organize a huge concert with the Chipmunks and use the proceeds for this purpose.
One idea could be Simon getting lost in a magical library and having to use his intelligence to find his way out while also uncovering secrets about the Chipmunks' past. Another could be Simon falling in love with a new character who challenges his intellectual side and makes him see things in a different way. Or perhaps Simon having to save the day when Alvin and Theodore get into a really big mess by using his scientific knowledge.
They could compete in a school - like setting. Imagine they are in a chipmunk academy and there's a big academic competition. Alvin is more street - smart and Simon is book - smart. They initially see each other as rivals in this competition. But as the story progresses, they realize that by combining their different types of intelligence, they can outperform all the other chipmunks.