The state-of-the-art of membrane technology is characterized by a number of mature applications such as sterile filtration, hemodialysis, water purification and gas separation, as well as many more niche applications of successful membrane-based separation and processing of fluid mixtures. The membrane industry is currently employing a portfolio of established materials, mostly standard polymers or inorganic materials (not originally developed for membranes), and easily scalable manufacturing processes such as phase inversion, interfacial polymerization and coating. Innovations in membranes and their manufacturing processes must meet the desired intrinsic properties that determine selectivity and flux, for specific applications. However, tunable and stable performance, as well as sustainability over the entire life cycle of membrane products are becoming increasingly important. Membrane manufacturers are progressively required to share the carbon footprint of their membrane modules with their customers. Environmental awareness among the world's population is a growing phenomenon and finds its reflection in product development and manufacturing processes. In membrane technology one can see initial steps in this direction with the replacement of hazardous solvents, the utilization of renewable materials for membrane production and the reuse of membrane modules. Other examples include increasing the stability of organic membrane polymers and lowering the cost of inorganic membranes. In a long-term perspective, many more developments in materials science will be required for making new, advanced membranes. These include "tools" such as self-assembly or micro- and nano-fabrication, and "building blocks", e.g. tailored block copolymers or 1D, 2D and 3D materials. Such membranes must be fabricated in a simpler manner and be more versatile than existing ones. In this perspective paper, a vision of such LEGO (R)-like membranes with precisely adjustable properties will be illustrated with, where possible, examples that already demonstrate feasibility. These include the possibility to switch properties using an external stimulus, adapting a membrane's selectivity to a given separation, or providing the ability to assemble, disassemble and reassemble the membrane on a suitable support as scaffold, in situ, in place and on-demand. Overall, it is foreseen that the scope of future membrane applications will become much wider, based on improved existing membrane materials and manufacturing processes, as well as the combination of novel, tailor-made "building blocks" and "tools" for the fabrication of next-generation membranes tuned to specific applications.