Allowing your party to plan their own mission

Based on some feedback from my players, I’m going to allow them to plan their next mission, which will be a ‘seemingly’ straight forward heist affair onboard a moving vehicle (limiting their geographical range).

What I’d like advice on, is how can I impose limitations on their plans to prevent them from going completely off-track?

Narratively, I’ll provide them with the objective and some key facts and stats, but what’s the best way to try and ‘plan for the unthinkable’ from my perspective? Normally as the DM, I can react to the team going off-road within the context of a quest because I can generally foresee the branches they might take, but in this instance, giving them the ability to map out their approach might make things tricky for me to manage.

Hope this question isn’t too vague!

Cheers