According to Roman Pichler,
A product roadmap is a high-level, strategic plan, which describes how the product is likely to develop and grow over the next months. This creates a continuity of purpose, and it helps product managers and product owners acquire funding for their product; it aligns stakeholders and facilitates prioritisation; it makes it easier to coordinate the development and launch of different product; and it provides reassurance to the customers (if the product roadmap is made public).
Roman Pichler – Goal Oriented Agile Product Roadmap
I like the Roman’s roadmap combines release goals and features. I have used this template on projects successfully.