Leia needs to die onscreen saving her son...

or someone important like Rey, since she might be revealed to be her daughter. It's best to kill on her onscreen in an honorable way to make the biggest impact on audiences. Make her the one who brings her son back to the light. Or she could be the one who sacrifices for the Resistance in a big battle during the climax or something. And since ep. VII starts with Han's funeral, it'd be great if it ends with her funeral.