The plumes I mentioned simply because it is on the same page. Looking further afield, there is no image of an Exeter crown in any form which is unquestionably distinctive, nor a whole host of other Civil War coins including the Bugle halfcrown, the Bristol flat-crown halfcrown, the York types 1-3 halfcrowns, nor the Tower mint group V halfcrown or most shillings from the same period. Some of these are major types, common too and all would benefit from illustration if identification is the reason for doing so. Then you have the regional die variations of the Saxon coins. The list is actually quite extensive because a picture speaks a thousand words. I don't disagree that it should be illustrated, but I suspect the same could be said of any period covered by the book.
The biggest problem has to be one of space versus production costs and selling price. A good number of collectors don't buy CoE because of the cost, preferring to have one of the cheap volumes, but that comes at a price with one listing all (100+) Oxford Civil War halfcrowns under a single heading for example. I note that this year there were fewer dealer adverts than previously which was a result of Phil Skingley leaving and the new editor taking over. These also take up space which could be better employed providing illustrations.
I also think there could be some mileage in splitting the pre-decimal into hammered and milled as well as having the decimal portion, given the number of collectors who restrict their habits to one of the three. It would be cheaper for the collector than the combined volumes which ought to increase sales.