“How long will your work be seen as current?”
I don’t know how common this question is in the viva, but I think it is a fantastic question to explore in preparation.
Get a piece of paper and jot down notes for an hour. Turn the question around in many different ways.
You have to think about the history of your discipline. What’s lead up to your research? You have to reflect on the value of your thesis. You have to see what is happening in your field, and think about how your work has been received so far.
You might put an estimate on the length of time your work will be seen as novel or useful. It could be that five years from now there will be something else that occupies the scholars of your field. That’s fine. Recognise that your work is a part of the unfolding story of research.
Your work makes a contribution to the sum of human knowledge.
(it sounds grand but it’s true!)