Ask any scriptwriter or a good storyteller and he will tell you that the plot comes first, everything else follows. It is so true for our corporate presentation decks as well.
Building your slides should be the tail end of developing your presentation. We need to first think about the main message, structure, its supporting points, practice it and time it—and then start thinking about your slides. The narrative gets build in the mind first and then it gets laid out on paper.
The presentation needs to stand on its own; the slides are just something you layer over it to enhance the listener experience. The slides are not presenter notes. It is far more effective when the slides are for the audience to give them a visual experience that adds to the words or better, the story!