Writing a book on Rome?

How can a writer not be inspired by Rome? Here's yours truly in a proud moment. I'm standing at one of the entrances to St. John Lateran, leaning against the giant bronze doors that once led into the Roman Senate. They were moved here in the 17th century to grace this massive church.

This poorly framed photo taken by some Italian kid reveals one of my original historical interests--the transition from pagan to Christian in the period of Late Antiquity. How did an empire that embraced a variety of gods and showed toleration to any religion that toed the line turn into a monotheistic force that changed the direction of faith forever? I covered this in my first solo book, Byzantium: An Illustrated History, and I'd like to get back into the subject.
Rome is a good place to study this because so many buildings from that period remain. Here's Santa Maria Maggiore, a fourth century Roman basilica converted into a church at a time when Christianity had only become the state religion within living memory. Some of the mosaics date back to 440! This was the first sight I saw in Rome and my biggest thrill. I've seen plenty of Roman sights, even hiked Hadrian's Wall, but all the ruins I had seen had been just that--ruins. Now I was actually standing inside a Roman building!
Not far away is Santa Pudenziana, with this beautiful fourth century apse mosaic. Within a century the Romans had gone from sacrificing to Jupiter, Mithras, and the emperor, to building churches.

This period is often overlooked in tours and guidebooks, and I'm thinking of doing a history/guidebook that would tell the story of the city's transformation from pagan stronghold to Christian capital. I'd tell it in straight historical narrative, but anytime a sight in Rome fits into the story, I'll switch into guidebook mode. Thus the visitor could follow the struggle between paganism and Christianity through Rome's buildings, while the armchair traveler could simply read it as a history book.

I've done a bit of research and I haven't seen any book that does exactly what I'm proposing. I'll have to look more before making a pitch to an editor and agent, though. There's a lot of books about Rome out there!