I agree with Sherman Heywood Cox II in this post entitled, "Do You Know Your Pulpit Blind Spots?" He suggests that devising a preaching plan is one of the best ways to prevent blind spots--over- or under-emphasis of certain themes, Bible portions, etc. He makes several other good suggestions. Read the whole thing.
From my earliest days in pastoral ministry, I drafted an annual preaching plan, to plan and guide my preaching a year at a time. It helped me achieve a better balance between Old Testament and New Testament, helped me touch on many themes I might otherwise have neglected, and guided not only my preaching but also my reading and study throughout the year.
Even with that, though, I must confess that, looking back, I still had multiple blind spots. For example, judging merely from the thirty-five sermon notebooks on my study shelf, which cover all but the last three or four years of my preaching (when I started storing sermons only on computer, not in hard copy), I clearly neglected much of the Pentateuch (especially Leviticus and Numbers), as well as much of Jeremiah and Ezekiel. I also neglected much of the Revelation until a 2010 sermon series.
In terms of themes, I preached a LOT on prayer, service, community, and mission, but not so much (at least since the early years) on the Trinity, heaven, hell, prophecy, etc. And, while the majority of my pulpit work over the years has been expository preaching, I sorta wish I had found a way to do a series on, say, the Apostles Creed, or something like that.
I also (much to my chagrin) did few character studies in my preaching. I preached through Ruth, Nehemiah, and Esther, and Jonah, and gave many sermons featuring David and Paul....but shared only snapshots of Joseph, Moses, Elijah, Daniel, Peter, and others. Drat.
So, yeah, I do wish I had occasionally taken an even broader view than my annual preaching plan afforded. But, on the other hand, that plan (and the many revisions and adjustments made to it on the fly) was the fruit of much prayer and listening and trying to discern what the Spirit was saying to the church. I don't pretend I always (or even usually) discerned his voice well, but overall, I think, I was better off as a preacher and the church as an organism to go where it seemed the Spirit was leading than to achieve a perfectly comprehensive survey of Biblical content and Christian doctrine.
So, what about you? Have you identified any blind spots or "pet" topics? What sort of measures do you take to prevent or address them? And what advice would you give to fellow preachers?