Another theme could be sacrifice. For example, Hershel might be written as being willing to sacrifice a great deal for Beth's happiness or safety, and this sacrifice is a testament to his love for her. Also, the theme of forgiveness might come up, if there are any misunderstandings or past issues between them in the fanfiction.
One place to find such fanfiction could be Archive of Our Own (AO3). Just search for 'Beth Greene and Hershel Greene' in the relationship tags, and you'll likely come across some great stories.