I think the key is just for it to be visual. I've found that if key ideas aren't accompanied by a visual, they don't get read or understood.
I'm not posing that this is the preferred or only way of creating a product roadmap. (I used this image because it's familiar with people when they think about product roadmaps).
I think a roadmap should be created based on the audience that's consuming it. So the answer in many cases is, "It depends."
We've been experimenting with gantt-style vs a simple table/list-view [1] and found that in many cases the latter actually works better in presenting the product vision, albeit at the expense of longer reading times.
[1] https://www.getshipit.com/blog/should-you-use-gantt-product-...