Heidelberg would make a nice stop for the castle complex,student town/oldtown.
It is on a different route than Rothenburg.Another nice town to stay on the same route is Schwaebisch Hall.

For me personally Wurzburg would make the most ideal stop for its residence, comfortable downtwon and festung/fort, however you can make this stop between Rothenburg and Frankfurt.

Frankfurt Airport Autobahn A-3, going east for about 38km as far exit #57 (Stockstadt). Head south on B-469 to the Miltenberg turnoff (42km from Stockstadt. Overnight in Miltenberg. It is an old town ala Rothenburg along the Main River (the Mason Dixon line of Germany), with castle you can walk up to. I last stayed at the Jagdhotel Rose at the beginning of town. Nice, but no elevator then.

From Miltenberg, return to the B-469 south and drive to Amorbach. Depending how early you get away from Miltenberg, visit the Monastery with its basilica in Amorbach. Continue on the B-469 to Wallduern and pick up the B-27 south to where the B-27 intersects with the Neackar River road. Turn left. Continue on the B-27 along the Neckar on the Burgenstrasse (Castle Road) to where it intersects with Autobahn A-6. This is 70km from Miltenberg. Take the A-6 east to the exit #53 (Lichtenau).

The following nice country route by-passes the Nuernberg traffic. From the exit, follow the road to Windsbach - Abenberg - Roth. (31km) In Roth look for the signs to Allersberg, Autobahn A-9 (31km from the Lichtenau exit). Essentially, in Roth when you come to the intersection with the B-2, turn left,and about 1km later, turn right to Allersberg. It is all well marked.

After you get onto the A-9, head south toward Munich as far as exit #68, the exit to the airport, north of Munich. Well marked. It is about 120km from Allersberg. Incidentally, the fields with the many telephone poles is where they grow the Munich beer hops.

Figure that on autobahns you will average about 110-120km/h, on the side roads about 60km/h

I fgure it will take you about 4 hours from Miltenberg to the MUC airport, with a couple of ohhh and ahhh stops.

PS: DAX's Heidelberg suggestion is also good. In that case, you would take the Neckar road/Burgenstrasse B-37 out of Heidelberg and continue as described after the B-37 merges with the B-27. I also agree on Wuerzburg as a good place to stop on the way to FRA. In fact, what I usually do when I visit family in Germany, I drive to Wzbg., drop off my car there and take the train express train directly to the FRA airport. Less hassle at the airport.

Look up train schedules on www.bahn.de, and read about the discounts if you buy the tickets early on the interent. It's all available in English, just "follow the flag"

If you don't have plans to get all the way Bamberg when you retrace to travel the Romantic Road, I can't think of a better place to stop than Bamberg.

And then "Schlenkerla" for the onion and a beer.

Woh, you guys are fantastic, thank you so much. I'm going to map all your suggestions out.

Thanks again.