Engineering mega-projects in Bangladesh, including large bridges, highways and power plants are marked by significant complexity, changing requirements and recurrent environmental and logistical challenges. Conventional project management methodologies, particularly the linear Waterfall model, have historically been employed to oversee these projects owing to their systematic planning and adhering to regulations. The inflexibility of Waterfall frequently constrains adaptability throughout project implementation, resulting unexpected delays, rework, and coordination difficulties. Consequently, worldwide project management trends have evolved to integrate Agile concepts, which prioritize iterative feedback, stakeholder participation and adaptability to change. This study investigates the effectiveness of Agile-Waterfall hybrid methodologies relative to traditional Waterfall approaches in the context of mega-engineering projects in Bangladesh. A conceptual framework is presented to examine the impact of the selected project management methodology (independent variable) on three critical performance indicators: flexibility, quality and delivery time (dependent variables). The framework emphasizes the moderating impact of organizational culture, managerial proficiency, stakeholder communication and technical integration. Literature indicates that hybrid models can reconcile structure and adaptability by implementing iterative enhancements within established phases, hence improving responsiveness while upholding compliance and documentation standards. Nonetheless, significant obstacles such as slowness to change, insufficient Agile training, administrative delays and inconsistent digital readiness impede effective hybrid adoption in Bangladesh.