Borges once claimed...
"Borges once claimed that the basic devices of all fantastic literature are only four in number: the work within the work, the contamination of reality by dream, the voyage in time, and the double."
James Irby in Jorge Luis Borges, Labyrinths: Selected Stories & Other Writings (1964).
I immediately saw a (possible) connection of this quote and the Ichnographia Campus Martius. Would it make a good opening quotation to Changing History? I guess the answer is yes and no. The voyage in time and the double definitely relate, and I suppose all the "sub-themes" could be considered the work within the work (but there really isn't anything like the city within the city or the plan within the plan), but I never felt that the ICM was contaminated by dream unless one considers Piranesi's architectural imagination to be a reality contaminated by dream.
If I were to use the quotation, I'd have to emphasize all four "devices" and how they manifest within the ICM, yet I've never taken the dream angle. Even if the plan is fantastic, it is nonetheless extremely ordered. Perhaps it is the plan/ichnographia itself that contaminated the reality of the viewer into thinking that they are seeing a dream (come true?). It seems that Bloomer more or less took up the dream angle.