I assume everyone knows of the relativization hierarchy by now.
1. A Second Relativization Hierarchy
Let's instead imagine a species with a language faculty that creates two relativization hierarchies, but also permits for a systematic exception.
The first hierarchy is familiar - the relativization hierarchy. I will not even modify it for this idea.
The second hierarchy is an "external" relativization hierarchy. It, and the first one, have implications between them. I will have the same order for that hierarchy:
Subject > Direct Object > Indirect Object > Oblique > Genitive > Object of comparative
What the second hierarchy tells us is which roles an NP of an external clause can be relativized as.
Thus, if a genitive in the main clause can take a subclause in which it corresponds to the oblique, then so can also the oblique, the indirect object, the direct object and the subject.
One could also imagine extreme things like "only the subject in the main clause can take relative clauses" or "only the subject in the main clause can correlate with anything but the subject in the subclause".
However, I imagine it could be likely for any NP to also permit an "echo" of its own role in the subclause, which would create a systematic exception. This type of exception I'd like to term a "linear" exception.
2. Subdivisions of the relativization hierarchy
One could imagine, for instance, that inanimate nouns have a stricter hierarchy than animate nouns.
3. Questions about the relativization hierarchy
3.1 Do we know where secundatives are with regards to the relativization hierarchy?
3.2 Is there any research or even any hypotheses around as to whether there's any roles that go to the right of objects of comparisons, or between the known elements?