I know that the Friedrichsbad in Baden Baden is a mixed naturist bath (I've been there), but I wonder if there are more mixed nudist baths, hot-springs, thermes or spas like this one.
The Schwabenquellen bath in Stuttgart, Germany is also a nudist bath, but they have one day a month when they require a bathing suit.
Other baths in Germany do it the other way around like the Schenkenseebad in Schwäbisch Hall (not too far from Stuttgart or Baden Baden). It has one day a month for people who like to swim in the nude.
In general in Germany most saunas in public baths are mixed and nudist. A lot of these places have indoor or outdoor pools within the sauna area that are usually nudist only. But you have to check individual places.