#2 gives you some excellent ideas. I would like to add by talking about the significance of the ending. Renewal is only made possible for the Samsa family through the death of Gregor. They have metaphorically used and abused him, and even his sister, with whom he had the most meaningful relationship with, has turned so completely against him that it is she who says enough is enough. I would also if I were you want to focus on the transformation in the family after Gregor's death and removal and how much better and brighter they are. This says a lot about what Kafka may be trying to communicate.
You have to decide what it is that you want to prove about parents and the relationships they have with their children. What is Kafka trying to illustrate by presenting the Samsa family in this way? The answer to that question will be your thesis.
After that, think about what you noticed about the relationship. Here are a few topics to get you started.
1. the lack of concern for Gregor's changed condition.
2. the parents not being honest about the financial circumstances of the family.
3. Gregor's sacrifices to his comfort in deference to his mother.
4. the mother's reaction to Gregor's physical change.
5. the father's abuse of Gregor with the broom and the apples.
6. the relief they feel when Gregor dies.