I have known that building of tunnel entrances through flat vertical wall (like in Locomotion) somebody already called. And it was rejected.
But my idea is not about it. My idea is about possibility of using of some tunnel entrances like additional flat land. With this effect of tunnels with entrances without slopes will be gained - but still in tunnel entrances would be diversity.
But of course, there is one problem - there would appear not only city building (res, com, ind) and then this covered tunnel entrance could not be re-built.