Variety: I'm sure you're already aware that you won't be hearing much French in Franconia. But aside from that, I don't think you gain much with the stretching of your itinerary to Colmar. Well, more tourists maybe, but then Rothenburg should already have tested your limits in that regard.
Old-world wine towns in Franconia to see along the Main River south of Würzburg:
Sommerhausen (just across the river from Winterhausen)
Area map (railway map but you can drive to these places too.)
Other area standouts that are on the same map:
Bad Windshem (with Franconian Open-Land Museum)
Bayreuth (east of Bamberg) with Margravial Opera House and the Maisel Brewery tour/sampling
East of Würzburg on the route to Nuremberg: Iphofen
Franconia, with the highest density of breweries per square km of all regions in Europe, offers many variations on this theme. Road signs like this one (Land der Brauereien = Land of Breweries) shouldn't be taken as an invitation to drink and drive of course, which is one of the reasons I have Franconian train maps in my "favorites". There are also bike paths in many places that connect adjacent brewery towns, probably a good way to burn off the calories and the buzz.
To the west of Würzburg, Miltenberg, the Pearl of the Main River, is a fine place too.
Should you in fact end up in the Alsace and lack the time to really see Franconia, I suggest swapping Würzburg for Bamberg, a more impressive place altogether.
Since you're leaving from FRANKFURT, I'm going to suggest that you spend at least half a day on the Rhine, possibly in Rüdesheim, which not only has a nice chairlift ride but would provide easy access to FRA airport on the day you leave. Why? Marksburg Castle. It's a short train ride or drive north of Rüdesheim along the Rhine River. No sense settling for the "rebuilt" castles you mention when you can tour this INTACT medieval castle.