Currently, Idol Manager only has two mods publicly available for download (although others have been worked on in the past, such as K-pop mod, but have since been made private). One of them is the Diverse Idols mod by chantolove, which adds new skin tones, hairstyles and dresses to the game.
Then there is this silly mod made by some guy who goes by the nickname Bicky which adds over a dozen new events, dialogues and substories to the game (with many more in the works!), as well as some new characters for you to meet. You can learn more about it here.
Some other users are interested in creating mods for Idol Manager, and are likely to share updates over at the Discord server. You can join it by using this invitation link. I myself have other mods planned for the future that are more focused on gameplay systems instead of storytelling (such as the Tax Mod), but that might take a while to see the light of day.