By far the best link between classical German literature and an entirely modern take on the original work is by the East German writer Ulrich Plenzdorf. His "Die neuen leiden des jungen W." says a great deal about the malaise felt by a younger generation in the GDR looking across the physical divide at a different world in the West, as well as exploring the heady sentiment of a young man in love which was the focus of Goethe's original tale.
For a very different feel,
you might want to try Carl Zuckmayer,
and perhaps his Der Seelenbraeu
(lit. the brewer or fulgurator of men's souls).
At one level his work, capturing Hessisch/Frankish parochial life is quite twee, but if you want to experience some non-standard German and a fairly realistic account of rural Hessisch life, it's worth adding it to your list.
In a similar vein, Theodor Storm evokes the tidal flats of the East Frisian with appropriate bleakness, and in such a manner that the parallels in German intellectual development
(late idealism, proto-existentialism, romanticism, naturalism) are manifest
There are passages in Das Siebte Kreuz, by Anna Seghers, which portray a slice of Germany which is alive with the human and the natural, as though 1937 and the gripping escape of 7 political prisoners into the landscape, the basis of the novel, could be just part of a timeless landscape being passed over by all-seeing clouds which have been passing that way for millenia, luckily unaware of what 1937 held.