The exploration of the Standard Model (SM) leptonic mixing has been led by the study of the neutrino (ν) oscillations phenomenon, whose discovery was acknowledged by the 2015 Nobel prize in physics. Half a century of experimental and theoretical effort has established and demonstrated consistency with the 3ν model and with the so far SM three family evidence. While no direct significant manifestation for physics beyond the Standard Model (BSM) has been found, the SM is known not to suffice to explain all today's observed phenomenology. In the forthcoming decade, most oscillation parameters are expected to yield sub-percent precision. Such a knowledge opens the possibility to experimentally test for BSM manifestation(s) via the direct and competitive exploration of the PMNS matrix unitarity for the first time. Any significant deviation might, in turn, evidence the existence of non-standard states (i.e. new neutrino) and/or interactions, thus allowing for direct discovery potential. Even if no deviations were found, the PMNS matrix structure, very different from its CKM counterpart, is of fundamental importance to our understanding of the leptonic flavour sector. In this document, we shall briefly review today's PMNS unitarity status in the context of existing and future particle physics programme within the next decade. We identify the possible need for a missing experiment(s). One such a case maybe a hypothetical Super Chooz project, employing the novel LiquidO technology, to address both directly sensitivity to the unitarity and unique impact to the exploration of the neutrino oscillation phenomena. Such a program is expected to additionally and coherently reinforce the physics of all currently planned experiments via indirect information aiding both the CP violation and mass ordering forthcoming measurements.