First, far in advance of submission, who would be good choices for examiners? Get their feedback, be a part of the discussion. Ultimately your supervisors will get to nominate.
Second, after the hustle and bustle of submission is done, what would they say is the most problematic part of your research? It won’t be anything that is wrong, and it might not be anything controversial, just something that needs a little extra thought.
Third, and once you’ve thought about it for yourself, what do they think are the strongest results or outcomes of your research? They will see everything you’ve done, but will have a slightly different perspective and that’s useful to know.
Finally, when the dust has settled and your viva is done, what can you do better now? There will be a lot of ways that you have grown during your PhD, but it can be difficult for you to spot them day-to-day.
What else might it be good to ask? Have a think, see where your ideas take you.