Something that would be a built-in method of displaying cascading (related) data in list view with a disclosure triangle (or similar) object or perhaps a choice of buttons. In effect a way to cause a portal to function as an appearing or disappearing layout part.
There are a variety of published workarounds to program this feature but it seems like it would be easier to develop into a solution if there was a new kind of layout part that would appear or disappear when toggled by a button, and that layout part would show related data the same way portals do. Perhaps this new layout part would be placed right underneath the Body part and then either a toggle button or a Specify calculation would cause it to appear or disappear.
I would like to see this as a feature that would only be available to users of FileMaker Advanced to build into solutions, but it would operate in FileMaker Pro, too. In other words, the new layout part is mot available in Layout Mode except in FileMaker Advanced.