Most complex products do not need a larger backlog. They need a clearer decision.
The useful question is rarely what can we add? It is: what must be true for the product to become easier to understand, use, and evolve?
That means looking at the business, the people using the product, the workflows between them, and the technical constraints as one system. Then choosing what matters now.
Good product direction creates a small number of choices that the entire team can move with. The interface comes after.