Relationship fields are resource intensive and hard to update through scripting. Partly due to the risk of a "cyclical" trigger error. IBPL fields reference an item in a relatively unobtrusive way, and display in a convenient text link format when not being edited. But IBPL fields can only back one type, and have many other restrictions to how they can be used. If we could have a new field type with the better attributes of both a relationship and an IBPL, it would open up many possibilities. This would be a one-way reference to another item.
For example, you could place such a reference on all content items so that they reference their own document. Content doesn't currently have a direct reference to its document, since the contains relationship is used for content sections. With a reference field you could set up FVA fields to values that are on the document. To do this with IBPLs you would need a separate field for each document type. And trying to maintain a single relationship on all content items has the potential for errors.
I would imagine this field could be edited like a relationship, but would appear similar to an IBPL. Or maybe there could be an option of having relationship-like editing or drop-down editing.
