One popular trope is the 'friends - to - lovers' trope. Given their pre - existing relationship as brothers in the original, it's interesting to see how writers develop their relationship into something more romantic. Another could be the 'hurt/comfort' trope, where one of them gets into trouble or is hurt, and the other is there to comfort and support.
One possible fanfic could be about how Simon's glasses are not just a simple accessory but a symbol of his intelligence. Maybe in the fanfic, the glasses get lost and the other chipmunks go on a wild adventure to find them, realizing how important they are to Simon.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, including Alvin x Theodore ones. Just use the search function on the site.
Perhaps in the fanfic, Simon's glasses are enchanted. One day, they start to glow and lead the chipmunks to a hidden library in the forest. Inside the library are ancient texts that only Simon can read with his glasses. The chipmunks then have to protect the library from a group of raccoons who want to destroy it.
The Chipmunks were created by Ross Bagdasarian Sr. in the late 1950s. He had the idea of creating characters with high - pitched voices. He used a sped - up recording technique to give the Chipmunks their unique voices. Alvin was the mischievous leader, Simon was the smart one, and Theodore was the cute and chubby one. They first became popular through a song called 'The Chipmunk Song (Christmas Don't Be Late)', which was a huge hit.