Mind the Product has a good write-up of C. Todd Lombardo’s talk called Roadmaps are Dead! Long Live Roadmaps! I’m also in the “prioritize themes, not features” camp:
Every theme on your roadmap should have a problem, an objective, and even some potential ideas you have to solve those problems. In practice, this changes how your roadmap looks. Rather than being a list of features, it might start from an objective of reducing support costs, and then show themes that might help reach that objective such as improving invoicing options or expanding payment types. By removing the details of what features will be built to fulfill these themes, it gives the team the freedom to figure out how best to solve the problems presented.
This is similar to some thoughts I had in a 2011 post called Product roadmaps are safe.