Parent pages are very helpful, but can only impact the very the bottom layer of the page it's applied to.
Would be helpful if a parent page could contain both layouts (base layer and/or an overlay layer), or the ability to apply up to 2 parent pages (base parent / overlay parent) to a page.

I'm sure every InDesign user who has ever used Parent Pages can see value in this feature:
Watermark graphics, common headers/footers/copy over changing images, etc.