How do you influence the product roadmap if the roadmap has already been laid out by product?
If you have relationships built with your product counterparts, and product leadership, you can influence through this relationship. This is especially the case if your product marketing org is accepted as a strategic partner instead of just a tactical partner. Ideally this partnership if founded on your PMs leaning on you to contribute as voice of the customer, data-driven insights on the market landscape, the competition, trends.
As the voice of the customer and market insights SME (subject-matter-expert), you can develop business cases and present them to your product counterparts at any stage, and more often than not, your product counterparts will listen and consider your recommendations. Even if the roadmap cannot be changed at that moment, that does not mean it cannot be influenced for the next set of iterations! And, product organizations are strategic in that most will build in wiggle room to address or adapt to changes in the market or customer wants/needs/motivations influenced by factors outside of their control–think Covid as the most recent example.
For organizations where product marketing is still being establish, continue to build relationships as well as your PMM muscles on bringing market and customer insights to the table, every single time. Become invalueable as a strategic partner and not just a tactical superstar!
I think the most important part of influencing product roadmap is that its an outcome created by a strong understanding of the user, strong understanding of the product and a strong relationship with product management. If you have those items already landed, then you can better understand if theres a gap that truly needs to be filled in the roadmap. If you lack one or more of those areas, then it may be hard to achieve the XFN buy-in required to influence roadmap.
To play back a sports analogy; imagine you're a golfer and you want to hit a longer drive. If you work on your stance for the right balance, and work on your core for power generation, and work on your pivot for optimal torque, and your swing plane and ball striking for impact--then you'll hit a longer drive and you'll probably also hit a longer 7 iron and pitching wedge. But if you go to the range and just swing your heart out, you might be trying harder to hit a longer drive but may not see results. The same applies to influencing roadmap--build a great foundation with your stakeholders, have great relationships with them, and in the end influencing roadmap will be much easier.
The great thing about roadmaps is they're like the Pirates' Code. "It's more of a guideline."
First, never lose sight of the fact that if you have a concrete roadmap from your product team (like really concrete, more than 6 months laid out), that's heaven for PMM. A vision for where you're going, commitment from engineering, and a view a few quarters out of what stories you get to tell? Sign me up.
Anecdotally, that's why I love the world of hardware so much. In networking, IoT, and even compute, you get a 12-18 month view of what's coming, and when. Getting that much runway to develop a POV, build a story over time, and test messages and positoning with customers? Doesn't get better than that.
But, just like a good sales cycle and customer priorities, there's room to flex. Features can be delayed or acclerated. Any good engineering team reserves 10-20% of their capacity for critical customer requirements. New capabilities can be tested and build to MVP to fit a market niche.
Learn your roadmap inside and out. Align and commit to it, and build the best story you can to fit the vector the product team is taking you on. And gather all the data you can on what's working and what's not. Through that, you'll earn the product team's trust, and then you can bring the data you need to argue for an adjustment.
Product roadmaps are, or at least they should be, always subject to change. At least that is what a PM will tell you when you want to share the roadmap with a customer. 🤣 So, use that to your advantage.
tl;dr
Make a case for change and back that case with hard evidence
Longer answer
I would suggest you start with a repeatable process for ensuring PMM has the opportunity to bring its value in the form of rich customer, market, and sales insights to the table when the roadmap is being updated. In the earlier days of Intercom that was quarterly. Each PMM would produce a "GTM Recommendations Report" for their respective product or product area, which summarized the top 3-5 "problems to be solved" (intentionally not "features to build"). Each recommendation was backed with qualitative and quantitative evidence as to why it should be considered for the roadmap. This evidence included win/loss data from sales, feedback from customer support, and of course a PMMs view of the market opportunity based on their knowledge of the competitive landscape. It was PMs' responsibility to take those inputs and provide a rationale on why something made or did not make the cut. The process has since evolved, but it was a successful starting point for improving how GTM and R&D worked together.
Lastly, a former team member and still good friend of mine, Jasmine Jaume, has a great post on this topic here. And, if you are looking to get that elusive "seat at the product table", check out my tips here.