paper architect

Someone whose work is purely theoretical, having never come to fruition, or has never been exemplified by anything in the real world.

"He was a tenured professor of architecture—a position buttressed by his renown as a 'paper architect.'
That is to say, he was an esteemed architect whose reputation rested on his designs, rather than on the buildings constructed from them."
—Asterios Polyp, by David Mazzucchelli