There are several techniques in use to produce a scope definition. Many of them involve constructing a product/feature matrix. One technique I particularly like is described in the book by David Weiss and Robert Lai about the FAST process for software product lines. They describe a commonality/variability analysis process that involves assembling key stakeholders and having them work out exactly what the proposed set of products has (or could have) in common, and the precise ways in which they vary from each other. This commonality/variability information then begins to feed important product line assets, especially the architecture that will have to serve for all of the products and support pre-planned variations.

There are other important steps to take when getting started with a software product line approach, but if you can establish the scope of your potential product line, you're already a long way towards engaging the right people in the organization and getting them thinking in the right direction.