What, ideally, should product marketing's role be in developing the roadmap for a SaaS company?

A successful product roadmap is one that is developed in partnership between engineering and product marketing. This allows you to both identify the trends/opportunities that will help you to grow your business and an understanding of how to speak to the value once it is delivered. Product marketing should bring to the table customer insights, market research, analyst insights, competitive landscape and business model implications to ensure that what gets built will allow you to maximize the opportunities identified.  

Product Marketing should closely partner with product and other cross functional teams (Eng, Finance, etc.) to inform and develop the product roadmap, including: 

• Insights: Provide key insights and synthesis from competitive intelligence, market trends, analyst insights, customer and Sales input, research / NPS, etc. Go beyond pulling all the information together to analyze it and provide recommendations.
• Storytelling: Product Marketing brings the roadmap to life through telling the story of the Product Strategy to take it beyond just a roadmap and shipping products, but why you are bringing these products to market. This is a valuable story to tell internally to gain alignment and buy in on the vision. And also tell the product strategy story externally to prospects and customers to get them excited about your vision and bring them along for the ride. 
• Target Audience: Identify the target audience to help inform and prioritize the roadmap. Identify who is the core audience you are building for. 

IMO PM creates products and PMM maps the product to the user. So I think the ideal role for PMM in shaping roadmap is to thoughtfully validate and critique the roadmap that Product creates based on your knowledge of the 1) user 2) product 3) market; with the ability to (aka having done enough homework to) defend the hypotheses you create for where your product should go. 

This question is already answered here, so check that out: https://sharebird.com/what-do-you-think-the-ideal-role-of-a-product-marketing-manager-should-be-in-roadmap-planning

People's responses vary based on their definition of Product Marketing at their company or in their experience. It's a fluctuating role, but my personal perspective is that Product Marketing should be part of the core team that provides inputs to help Product teams understand priorities from the market.