Marty Cagan's post on the topic tries to do both, while at the same time presenting a better way of achieving the underlying goal. (link the alternative to roadmaps SVPG).
My view on this is that a product roadmap usually repeats that old problem of design versus need.
State the requirement in terms of the problem or goal that you want to address, rather than in terms of how you think it should work.
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The essential character of a product roadmap is, ultimately, yet another GANTT chart (YAGC; a pretty horrible acronym after all).
