Keynote Master slides can unfortunately only have one main text box on them, so you will not be able to have a standard Master template that does exactly what you want. However, you can always add additional text boxes to individual slides. What I would recommend is setting up a slide Master with the bullet points on the right 2/3 of the slide, then using this Master create a new individual slide, and add a text box on the left for the main idea. When you create additional individual slides from the Master, simply copy this text box and paste it on to your new slides -- it will appear in exactly the same place on each slide.