What do you think the ideal role of a product marketing manager should be in roadmap planning?

I think it depends on what the priorities are at the company and the other roles pre-existing (they both affect what the PMM needs to do more of), but I think the PMM contributes input for the roadmap based on the market feedback they are in tune with. The pmm can also do whitespace evaluation in the market.

Depends how market-facing your PMs are. If they're obsessed with serving customers and creating products and features that will generate new business, probably don't need to be very involved.

If they're focused on internal things though and are more interested in how engineering will implement something vs. why - that's where PMM needs to be very involved

No matter how market-focused your Product Managers are, they still have to balance market needs with technical requirements. Product marketers can help by providing quantitative data around what customers and prospects are asking for across the product lifecycle, analyze the product gaps coming out of that feedback, and measure the impact to sales and retention. Here's a good resource for how product marketers can use voice of customer trends to guide and influence roadmap.

I agree with a lot of the other answers, if there is not a lot of voice of the customer in the discussion, it can often be the role of a PMM to insert it. I'd also recommend you talk about the timing and how it might impact go to market launches, for example, if there are 3 of 15 features that will be launched over the next 6 months that can be packaged together to tell a better marketing story, then I'd suggest to the PM team to group them together to make for a more buzz worthy launch.

Market research / gaps / competitive analysis / analysis of market value of new features / products. they would work hand in hand with PMs to consolidate all the info into a true roadmap.

As we move into a product-first world, the best companies have cross-functional teams designed to drive user engagement and success. Product Marketing is a key component of this. 

Roadmap planning is a crucial part of getting to product-market (or product-channel) fit and I believe Product Marketers have a key role to play in this. In fact, this is how I would define Product Marketing: 

Image source: A New Definition of Product Marketing

This means that Product Marketers have as much of a role in defining (shaping) the product as they do in communicating the product. Having a balance will mean that this becomes a virtuous circle and speed up the time to Fit. 

This may not be a standard definition (yet) and although there are many different flavours (and a lot of inconsistency) in defining Product Marketing across different organizations, it's helpful to have a holistic understanding of what the role can (and maybe should) include, especially as we collectively mature in our understanding of how best to treat this role amidst our teams.