A few days ago, Random linked to a product roadmap for Firefox, a web browser. A product roadmap shows the expected changes to a product over time.
Product roadmaps can have different audiences, and I believe the format of the roadmap should depend on the audience a product manager is trying to target. One common format for a roadmap shows features that the company plans to add to the product. Users and analysts are accustomed to thinking about products in terms of features, so to some extent this format is helpful and natural.
However, particularly if the audience for the product roadmap is internal, it might be worthwhile to orient it around use cases rather than features. The high-level use cases for a product typically don't change much during its lifetime. Yet the constraints around these use cases do change; even if the use cases stay the same, you make the product easier to use, more secure, or more reliable.
One way to format your product roadmap, therefore, is in terms of the use cases and the enhanced constraints you will attach to them as the product matures. How much easier will it be to use your product, in measurable terms? How much more secure will your product be, in measurable terms? How much more reliable will your product be, in measurable terms? You don't even have to mention the features that will lead to these improvements.