As opposed to a portal object on a layout, this would be a layout part on par with Header, Body, Sub-summary, etc.
This part would be like a Body part in list view, except that in the Part Setup dialog you would specify the table occurrence to display. This part would repeat for every related record.
Why This Idea Is Important To You / Use Cases
This part would allow developers to build more complex reports that could draw from multiple tables. You could report on invoices with n line items from the invoices table instead of going into list view in the line items table, for example. Slightly more complex: you can print a recipe with its list of ingredients and a list of method items from the recipes table.
The current workarounds for this sort of thing are: 1. report from the parent table and include portals to each related table (which limits the number of related records you can display and causes pagination problems), or 2. build a virtual list report, which is powerful but requires a lot more work.