Gosh, what a question, Gil. What is your answer?
I have written many books in my head, but I am much too lazy actually to write a book. I guess my first choice would be
Geometric nonlinear functional analysis, volume II
and my third choice
Geometric nonlinear functional analysis, volume III
neither of which will ever be written by the authors of volume I.
A less daunting topic, until you think about it a lot, is
The geometry of $L_p$ spaces.
Another one I toyed with was
Approximation properties of Banach spaces.