Engaging 'abused Wally West fanfiction' can be achieved by building tension. You can do this by showing the power dynamic between the abuser and Wally. Show how he's trapped in the situation at first. Then, introduce elements of hope gradually. Maybe he discovers a new power or meets someone who gives him the courage to fight back. Also, explore different aspects of his character that might not have been shown before, like his hidden strength or his vulnerability in a new way.