Slogging across the north German plain doesn't do it for me, either, I'm afraid.
The distances are too long (I learned from experience riding part of this with Uta in August, although they are flat, progress is slow on the Dutch and German cyclepaths, it took longer than I had thought and we were finishing at 8pm after 12-hour days - with only short stops and no mechanicals - which was too late).
Also, it's not the pretty part of Germany. We're not the only people to spot it's the easiest route to Berlin: most of it was flattened in the war and it was rebuilt afterwards in functional but plain concrete. And, while Berlin has appeal as a place we've all heard of, cycling into a big capital city is never great. It tends to mean an unpleasant half a day on busy roads.
I came on here to post two suggestions.
Lee and Jenny have already made one of them; to do a longer thing in the Netherlands / Belgium.
If anyone hasn't done it, cycling in the Netherlands is pure joy. The small Dutch towns are stunning and the cities aren't bad either. Those bridges and dams in Zeeland are as breathtaking as the alps and a bit easier to cycle over (especially if you get an echelon going into the crosswind). Transport is easy at both ends. Even the food is ok nowadays - it not quite as cheap as Germany.
I did a loop from Hoek to Leiden, Haarlem, Amsterdam, Weesp, Zeist, Utrecht, Gouda, Delft, Hoek, a few years ago. That was about 40 miles a day. Would be easy to extend it, to go up the coast a bit more, up towards, the Friesan Islands and back via Edam, or even over the long bridge up towards Groeningen, or south into Flanders, where the cycling is second only to the Netherlands and the delights of Bruges, Ghent, Antwerp await with their pretty canals, world-beating beer slections, moules-frites and countless other temptations.
My other one would be to go to south-western Germany and do a route there. For example, the Romantic Road.
Here's a site that a CTC group who were there a few weeks ago. Villages are stunning, easy cycling (we could extend their distances a bit), Alpine views without the big climbs, etc. All the ingredients are there to construct something a bit more ambitious. It's got the Rhine and the Danube, the main rivers of Western and Eastern Europe, the Alps, the Black Forest, and the meeting point of France, Switzerland and Germany at Basel. Switzerland is a joy to cycle in, and the northern bit is flat but with lovely views of the alps. I've not thought out an itinerary there, as I'm not sure if it would pass the 'must be accessible by train' test, but it would not be hard to put together something spectacular from that material.