This one is very simple. Make a game where the main character of the franchise is NOT present. For example, if you're making a game based on the Sonic franchise, make a game where Sonic himself is nowhere to be found. You can mention him in the story, but he's not allowed to appear in person.
We won't be too strict on the criteria of a main character. If it's something up to debate, we will allow it. Nobody's going to deny if Sonic is the main character of the Sonic franchise. But Tails? I guess you could make an argument that he is one of the main characters, but we will allow it regardless because you could also argue that in a vast majority of the games he's more of a sidekick.
PRO TIP: Remember that the absence of the main character doesn't just have to be a narrative choice; it can be a driving force within the gameplay itself too. Don't just make a typical Mario game and replace Mario's sprite with Luigi. Make the gameplay all about Luigi! Make him do something only he can!