Here's two other possible suggestions, although I don't know how convenient either is by train from Augsburg.
If you have a much greater than normal interest in the Napoleonic conflicts, you can consider a visit to the Befreiungshalle above Kelheim. Don't bother, though, if you don't obsess over this kind of stuff. The town of Kelheim itself is nice enough, but nothing too spectacular.
The other option, which I would recommend much more highly, is Landshut. I won't go into too much detail, but let's just say that I visited Landshut the week before I saw Salzburg for the first time, and I can honestly say I was far more impressed with the former. Both have a large castle overlooking the town, both are built in a similar Baroque style, but whereas Salzburg is kind of monochromatic, Landshut is a burst of Technicolor. The views of the Alps in Salzburg, though are far better (you can only barely seem them from certain vantage points in Landshut when conditions are perfect).
I spent a few hours in Nördlingen once per Lee's frequent recommendations and I was quite impressed. Very similar to Rothenburg, if there's a bit more modern buildings, but none of the trinket shops or tour buses that have turned Rothenburg into a hollowed-out tourist funhouse.
I've only driven through Landsberg. Looked nice enough, but I didn't get a closer look.