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 gives the whole team a small number of choices to move with. The interface comes after.