I've proposed this on the official SImCIty 2013 forum, back before I fully realized that electronic arts is full of dumb morons.
Here is my idea: Medium Density Avenue with streetcars. As in where bridges have the sub-menue of changing the bridge to arch but keeping its density, give Medium Density Avenues a sub-menue to include streetcars that way, if someone like myself wants to have a college town, with all the medium density buildings and such, and the green meridian in the medium density avenues. Why not have streetcars? That way, in order to fully maximize the potential of ones city via mass transit, you don't need to fil your nice little college town with concrete jungle, which would make more traffic and make the whole reason of why you put in the streetcars moot.
Another idea involving this, why not have a mini great works project where two cities can add streetcar tracks on the freeway between their cities? I've seen it IRL, in the L.A. area, there is a train system in the middle of the freeway, linking downtown to L.A.X.