2 weeks

Here's a couple of suggestions between Frankfurt and Berlin. But first, answer this question: do you plan to rent a car or rely on public transportation?

My first recommendation would be the northern edge of the Hartz mountains, particularly the towns of Goslar, Wernigerode and Quedlinburg. The hike to the summit of the highest mountain in the region, Brocken, is particularly popular and not difficult at all.

Eisenach and the Wartburg castle is another good stop, although I probably wouldn't spend the night here.

If you want to see something like Rothenburg, but don't want to head south out of your way, consider the walled historical town of Fritzlar in the Waldeck region of northern Hessen. This region also has a nice liberal sprinkling of castles, at least one of which, Felsburg, remains remarkably well maintained. Without a car, though, this region is out of the way and difficult to explore.

22,

Care to give us a hint on budget, preferably in Euros.

also any hint as to what you would like to see/do or not do and not see?

happy trails.

Here is a "Romantic Road" from Frankfurt to Berlin (all perfectly preserved towns, just mostly without town walls):
Frankfurt - Büdingen+ - Alsfeld -Schmalkalden - (Thuringian Forest) - Eisenach (Wartburg castle++) - Erfurt++ - Weimar+ (from Erfurt) - Bad Langensalza - Mühlhausen - Duderstadt - Stolberg+ - Quedlinburg++ - Wernigerode+ - Goslar++ - Wolfenbüttel - Gardelegen - Tangermünde+ - Brandenburg an der Havel - Potsdam++ - Berlin

And this is a more Eastern route. Less "romantic", but more valuable towns, more nature, and more diversity:
Frankfurt - Büdingen+ - Alsfeld -Schmalkalden - (Thuringian Forest) - Eisenach (Wartburg castle++) - Erfurt++ - Weimar+ (from Erfurt) - Annaberg-Buchholz+ - Seiffen++ - Freiberg+ - Dresden++ - (Meißen+) - Saxon Switzerland++ (+ Pirna and Königstein fortress+) - Bautzen+ - Görlitz++ (maybe Zittau Mountains with Oybin+) - Bad Muskau+ - Spreewald++ - Berlin

"+" = deserves a detour

"++" = must see

Plan is to rent a car in Frankfurt and keep it for about 10 days, travel south and then east. Per Rick's 3 week agenda, drop off rental in or near Wurzberg and train to Berlin.