Allow users to specify the order in which linked entries are returned from the API. The UI could handle this via simple drag & drop ordering.
This is the holy grail for our project. We use relations in many places: to manage complicated content structure within a page where a rich text field may be insufficient, to link re-usable content (such as office information), to define entities in our navigation, to specify related products, as some examples. It is a terrible burden to have to define sorting tables for each and every one of these scenarios.
Ordinal relations are a critical data modeling tool; nearly half the relations in my active projects need it. I currently fake it by adding an extra model layer to hold the relation and the ordinal, but that is very brittle, and also hard to train users on.
Really need this!