Thanks, @StgConti & @TiAm.
Alternatively, or a variation of the old/young Sandy... Have Sandy B be the main character, but have Sandy A be the "unlockable" character where aspects of her past can be played in-game and detail parts of her story only hinted at in the main game. Like, let's say her main story hints at why she got into the superhero business... Well, somehow you can unlock a special level that lets you play as Sandy A to explore that sub-plot, thus adding replay to the game - finding hidden stuff - which also opens up deeper content of the character. People - like me - will eat it up like Cap'n Crunch on Saturday morning while watching cartoons in my pajamas.
I'm seeking Narnia. Who wants to come with me! Aslan is on the move!