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.
I hope not. Incestuous relationships are not acceptable in society and are quite disturbing. Fanfiction should also adhere to basic moral and ethical boundaries.
Definitely not. Abuse is a serious and often triggering topic. Even in the context of fanfiction, it can be very intense. It may not be appropriate for general audiences as it can cause discomfort, anxiety, or even trauma for some people who read it, especially if they have had personal experiences related to abuse.
It would be very controversial. It would likely face a lot of criticism from the fan community and those who uphold moral values. Such a concept is against the natural order of family relationships.
Daryl and Beth have very different personalities. Daryl is the tough, brooding survivor, while Beth is more of the gentle, hopeful one. This contrast makes for an interesting dynamic in fanfiction. Writers can play with how they interact, support each other, and grow closer in the difficult world they live in. It gives a lot of room for creativity in exploring their relationship further.