Berlin to Frankfurt by car

Don't bother going out of your way to Rothenburg, there's more than enough good alternatives along your route. Some suggestions that haven't been mentioned yet are Quedlinburg, Magdeburg, or Eisenach. Although I wouldn't spend the night there, if your chosen route makes it convenient, consider a brief stop-over in Friztlar. More or less a slightly smaller but much less touristed alternative to Rothenburg o.d.T. Along the drive to Friztlar from the A7, you will pass several castles. Felsburg (overlooking the town of Felsberg), is nearly intact from it's medieval state.

Hi, I also suggest Weimar, Magdeburg (for a larger place after leaving Berlin), and Lutherstadt Wittenberg too, which I only saw from the train...looked very interesting. Your choice of Erfurt to spend one of the nights is good. Skip Rothenburg.

Actually Wittenberg is only an hour from Berlin so I'm not sure if this already qualifies for a stay over. But the town is lovely and historically unique. You may want to spend half a day at the Wörlitz Park (if you like 19 century European parks and grand gardens). I would stay two nights in either Weimar or Erfurt for all the sights (including Wartburg castle in Eisenach, another Martin Luther sight). The Krämer bridge in Erfurt is like the old London bridge or the bridge in Florence. After Erfurt you could also spend another night in Würzburg - or Fulda and visit the Röhn national park.

Or you could spend 2-3 days in Frankfurt. I enjoyed the city tremendously.

I'd strongly suggest a day to travel through the Harz, with a stop on Werningerode.