How do you structure a stage plan to keep track of the stage goals?

The following are suitable as top-level organisational criteria (also called EPICs)

• High level functional architecture elements
• High level technical architecture elements
• Teams
• Sub-projects

…or a mixture of these, if it helps.

Agile teams should provide a forecast of goal achievement from their sprints.
Conversely, the project plan should help structure the stage plan. We need a “linking element” that can be found in the world of the project plan and that of Agile product development: the EPICs.