Although approaches that effectively address the distribution and dynamism of adaptive systems at a middleware level exist, the design of complex, ensemble-based systems still remains a significant challenge. This hinders the development of real-life applications based on the ensemble paradigm. A promising approach appears to be the coupling of proven low-level concepts with high-level ones, revisiting requirements modeling in the realm of ensemble-based systems. To this end, the goal of this paper is to point out the specific challenges related to the design of ensemble-based systems and show that classic requirements models and methods cannot be applied out-of-the-box in a requirements-driven design of ensemble-based applications. In response to this problem, a novel design method based on the iterative refinement of system requirements expressed by predicates on stakeholder's knowledge is discussed.