At the time I went to my viva, just over eleven years ago, I didn’t know for certain that I was finished with my research. I was 90% sure that I wasn’t going to get any kind of academic position, 90% sure that my research was done and I wouldn’t do more. And yet I had to be ready to talk to my examiners about what I could do with my research, or rather, how someone else could continue the work.
I talked about special cases that might be of interest. The utility in making a better computer program of an algorithm I’d developed. Other problems where my methods might be applicable.
But I was clear: I might not be doing this. These are just ideas.
You might be in a similar position. Or perhaps you know that when you’ve finished your PhD you’re leaving academia. You don’t have to lie, you don’t have to fib, but I think you do have to have something in mind. There are many reasons why people leave academia after the PhD. But there’s a natural line of questioning in the viva, “You did this, what could you do next?” Even if your examiners look to you with an expectation that you’ll tell them of your plans, you can reframe that sort of question with general ideas, as detailed as you think appropriate.
Possible future plans are just that: possible. Start with why something might be a good idea, say how someone might do it, what they might do. But you can be clear, if you want to, that that’s not the road for you.