Defining and containing scope is always a difficult part of any project.
One method and perspective for handling scope is capabilities and the traceability of those capabilities to overall business objectives.
The first step is defining what the business objectives that business is trying to achieve. The overall business objectives will range from objectives that fulfill the strategic plan or vision to more pragmatic objectives.
A strategic outcome would be outcomes such as increasing revenue by 10% or decreasing time required by 10%.
Pragmatic outcomes would be outcomes such as retiring an existing application.
Starting with the business objectives allows the scope to be defined in a manner that ties it to values that is being created for the business.