From a quick glance at previous volumes, I'd say what still needs to be covered is EMD SD35, SD39, GP38-2, and SD70M; post-U series GEs; Alco switchers; and GE 44-tonners, 70-tonners, and miscellaneous small units (Plymouth, Davenport, etc.). Perhaps an updated Cotton Belt book, as his original came out in 1977.
Although not strictly motive power, it would be interesting to see a book documenting operation on the "Cotton Rock" and Chicago Missouri & Western (St. Louis-Chicago) taken over by SP in the 1980s.