There are things, as long as you are considering them, that can only be done during the construction phase and cannot be changed afterwards (and if they can, only with considerable effort). I am thinking of flooring, doors, light openings, and so on. Things that can be retrofitted are therefore pretty low on my list. And some highlights had to disappear completely because, from our point of view, the cost-benefit factor was not convincing. And so everyone has an individual list of things they want or have.
To the question, what do you really need?
My answer: a few walls, windows, a door would also be quite smart, and a roof.