What's underneath the roof deck? What size are the ceiling beams? What would you do about the vents?

You have to find a spot for those vents. That's number one. Decks are made to hold the weight of the people walking on them. sitting on them, throwing beads off them. Roofs are not expected to be an entertainment platform. Decks and balconies are usually not sloped unless you want to recreate the Sky Cells in Game of Thrones. What will you do to direct water away from the house and into the gutter? What sort of safety railing is required per your building codes? How will that add to or detract from the roof line?

It could be done- most things can. Is it worth doing? Do you have the budget to do it? Obviously the door pre-dates you so someone thought this was a good idea.