Lot's of myth floating around about requirements elicitation and management. Starting with requirements is not how large, complex, mission critical DOD and NASA programs work. Start with Capabilities. This cna be directly transferred to Enterprise IT.
Here's a map of the planned capabilities for an ERP system. This figure is from Performance-Based Project Management® where all the deatails of this and other principles, practices, and processes needed for project success can be found.
Here each business systems capability is outlined in the order needed to maximize the business value. In agile parlance, the customer has prioritized the deliverables. But in fact the prioritization is part of the strategic planning needed to assure that the cost investment returns the maximum value to assure the business receive maintains a positive ROI throughout the life of the project
The first step is to identify the needed capabilities. Here's the steps
Only when we have the capabilities defined for each stage of the project can we start on the requirements. All the capabilities need to be identified and sequenced, otherwise we can't be assured to business goals can be met for the planned investment. With the planned capabilities, we can start on the requirements
With requirements in place for each capability, we can then start development. This is done incrementally and iterative using our favorite agile method. Doesn't mater as long an incremental delivery of value of the approach.