Roadmap themes represent the underlying problems you really want to solve. It takes a savvy Product person to tease these ideas out of the noise of requests from every quarter.
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Smart question: "What would we need to be true for our product to realize its vision and meet our business objectives?" Expressing problems clearly is highly effective at guiding the development of solutions.
UX maven Jared Spool has a helpful podcast on themes. (I'm quoted; I get around). Is your organization making the problem clear enough before debating solutions? What's a dream theme? Discuss.