This is a question about a structure that can be used to investigate all kind of structures that can be investigated. Many years ago with Joseph Gubeladze we discussed something similar but I only remember it was something more profound than what I am able to ask now.
I'm afraid I cannot say anything sensible about it. All I know is some amount of ($n$-)categorical semantics for type theories, which inspires some vague suggestions like writing down axioms for the relations between entities $a$, $b$, $c$, $d$, ... with the semantical meanings like "$a$ is the theory of proofs of $b$ in $c$", or "$a$ is a description in $b$ of an interpretation of $c$ in $d$", or, in type-theoretic context, "$a$ is a term of type $b$ in the type theory $c$", or, in category-theoretic context, $a\in\hom_b(c,d)$, or $a=\hom_b(c,d)$, or ...
Did anyone write down such axioms and use them to study some properties of such "abstract theories"?