I would ask them what their goals are for this game - what makes it a success/failure.

I would ask them how this game is supposed to feel. I would ask them about how important verisimilitude is to them and how they plan to achieve it.

I would ask them how they intend to use playtesters and other sources of information and how that feedback gets incorporated into the game.

I would ask them how they decide to use 5e RAW vs home-brewed rules.

I would ask them what they think they are doing well and what they think they are doing poorly. I would ask them about their plans in light of the answer to the previous question.

And then I would probably have a few hundred follow-up questions.