While I would very much appreciate any work in this direction, I believe
the design should start not in the frontend (and thus be concerned by
syntax), but instead in the middle-layer or backend. (The backend may
actually be easiest, especially if we focus on SQL-enabled backends.)

Using such an approach helps to define a robust and expressive grammar,
and reduces the danger of running into a wall, where it is hard or
impossible to formulate new compound expressions.