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.