Germany has a rich cultural heritage that would make for a great stamp set. Kant, Hegel, Schopenhauer, Nietzsche, Goethe, Hesse - to name just a few - would all deserve a stamp. Also because their influence in the world is far bigger than that of the fairly poor German cuisine that currently represents Germany on Slowly.
Similar ideas have already been implemented, see the Greek and Austrian sets.
Here's a list of candiates that would be great to see in stamps:
Slightly more controversial candidates would include Karl Marx (https://en.wikipedia.org/wiki/Karl_Marx) and Martin Heidegger (https://en.wikipedia.org/wiki/Martin_Heidegger).
Please feel free to add more or discuss my suggestions!