Yes do not miss either Luxembourg City or Strasbourg en route to Colmar - Strasbourg is IMO a vastly underrated city - nice canals, smashing historic old church and just a pleasant pleasant city

Luxembourg City itself is not much but its location is stunning - a deep gorge bisects the town center, making a wondrous vista. Take a tour thru the casements - tunnels hewn out of the cliffs thru the ages for military reasons.

And also do not miss Trier, one of Europe's oldest towns and once capital of the western part of the Holy Roman Empire - there are fine Roman relics like the Porta Negra, the gateway to the eastern part of the Holy Roman Empire but built by the Romans and IMO the finest intact Roman relic north of the Alps.

Trier also has a Roman temple that is now a Christian basilica, a Roman theatre and the Karl Marx House where the philosopher once lived as a kid.

Once in Colmar take a short trip along the Route des vins Alsace - the Alsace wine road that meanders along the foothills of the vosges mountains and passes thru many vineyards and by wine houses where you can sample the wares and buy direct from the consumer - the best part of the road I think lies just north of Colmar - Riquwehr (sp?), Ribeauville (sp?), etc.

to get to Rothenburg I would recommend as one of many possibilities going via Heidelberg, one of Germany's few large cities not smitten by the Allies in WW2 and one with a fine fine castle hovering high over it and from Heidleberg

taking the Castle Road along the river that goes thru Heidelberg - whose name escapes me all the way to Rotheburg - castle-studded river valley with several neat old towns to putz around it on the way.

http://goeurope.about.com/od/mapsofgermany/l/bl_castlerd_map.htm

I believe the river from Heidelberg you are referring to is the Neckar river (with some castles along the way)

Actually re-thinking the last 2 nights and may not stay in Rothenberg, but stay in Heidelberg instead for 2 nights, spending 1 day in Heidelberg and 1 day along the Neckar, then driving to Frankfurt airport directly from Heidelberg. any pros or cons with that plan? or any other tips for this area?

seems a nice plan to me! Speyer makes a nice stop on the Rhine en route to airport - uber famous cathedral and palace or something right on river in this cutesy nice pleasant smaller city.

We did this journey about 4 years ago. (twice in two days with a week in each area at the end, don't ask but it was required as we were picking up wine)

Luxembourg/Trier/Traub-Traubach/Colmar/Strasbourg are the big drawers

But Bad Durkheim has the world's biggest wine festival going while you are there (do not go there and try to drive as tasting samples are 0.4 litres).

We will be in Cochem in Sept as well. We will have 13 days and debated between going to Alsace and Luxembourg or heading east to Rothenburg, and visiting the other "bergs". Report back on your visit to Strasbourg,Colmar and Luxembourg. Those places on our list to see next time.

Our itinerary looks like this:
Days 1-2: Cochem (Mosel Valley)
Days 3-4: Bacharach (Rhine Valley)
Days 5-6: Heidelberg (Neckar River Valley)
Day 7: Drive castle road stop in Bad Wimpfen overnight
Day 8: Stop at Schwabisch Hall and Dinkelsbuhl en route to Rothenburg
Day 9-12: Day trips to Bamberg, Nurenberg, Wurzburg from Rothenburg
Day 13: Frankfurt and then home

Have a great trip!