The rationale behind using sub-objects was that not all playbooks would have layout parameters defined. And it was easier to ignore/add content for layouts in playbooks when its an object with parameters instead of a list of parameters. If you make them flattened list of parameters then it introduces a level of complexity from an understanding / use perspective if you have playbooks with layout vs playbooks without layout.
If we keep the list of parameters for layout instead of sub-objects then please make sure that it's clear those specific parameters are only relevant if using the layout feature. It is insufficient to just mark them as optional.