  The central goal of crystal engineering is to develop precise control over
material function \emph{via} rational design of structure. A particularly
successful realisation of this paradigm is the example of hybrid improper
ferroelectricity in layered perovskite materials, where cation vacancies and
cooperative octahedral tilts combine to break inversion symmetry. However, in
the more chemically diverse and technologically relevant family of inorganic
ABX$_3$ perovskites, symmetry conspires to render hybrid coupling to polar
distortions impossible. In this study, we use group-theoretical analysis to
uncover a profound enhancement of the number of improper ferroelectric coupling
schemes available to \emph{molecular} perovskites. This enhancement arises
because molecular substitution on the A- and/or X-site significantly
diversifies the range of distortions possible. Not only do our insights
rationalise the emergence of polarisation in a number of previously-studied
materials, but we identify the fundamental importance of molecular degrees of
freedom at the A-site, which are much more straightforwardly controlled from a
synthetic viewpoint than are lattice, charge and orbital degrees of freedom. We
envisage that the crystal design principles we develop here will be enable
targeted synthesis of a large family of new acentric functional materials.
