Rapid socioeconomic and climate change pose challenges for reservoirs to reliably meet water demands and reduce drought impacts within increasingly interconnected Energy-Water-Land (EWL) systems. To inform strategic planning of infrastructure, we explore future pathways for global reservoir storage capacity expansion using our newly developed Global Reservoir Yield (GLORY) model, two-way integrated with the Global Change Analysis Model (GCAM). GLORY optimizes reliable water supply levels and costs from reservoir storage capacity, considering intra-annual climate conditions, reservoir storage exploitable potential, and dynamic water demand. We analyze a set of climate scenarios from the Coupled Model Intercomparison Project Phase 6 (CMIP6) combined with Shared Socioeconomic Pathways (SSP), representing varying water demands, population and GDP growths, and technological changes. Preliminary results reveal strong regional heterogeneity in reservoir expansion pathways, with China, South Asia, and the Middle East and North Africa projected to require the most significant capacity expansions across all scenarios by 2050. This work advances GCAM-centered multi-model coupling to better capture interactive responses of water storage and its multisector implications under uncertain climate and socioeconomic changes, providing critical insights for multi-sector planning, water resources management, and infrastructure investment.