Rue de Rennes
Rue de Rennes, slicing northish from around Montparnasse to Boulevard Saint-Germain. Planned in the mid-19th century, it was originally supposed to reach all the way to the Seine. But you know how it is in city planning, what with one thing and another, only its first two stages were constructed. And maybe just as well, since its final stage would have meant demolishing a number of historical buildings—including, if I’m reading my map right, the extremely historic and still active Institut de France.