I've found ballet to be very helpful. It really helps you to develop your balance and use over your legs and feet. People may say ballet is airy, but to do a basic ballet class you need to be very grounded, like in ballroom, or you will fall all over the place.
Some ballroom studios in my area teach ballet classes because there are so many adult beginners doing ballroom who would like to supplement it with ballet. Ballet will also help you to develop flexibility through your torso, good use of the arms, and awareness of how the head and spine relate.
I find it to be excellent cross-training, especially if you are limited in your partner practice time due to Pro/Am.
Another good form of cross-training is taking body awareness/movement classes in such things as Alexander technique, Feldenkrais, or Pilates. Of course, the utility of the work is going to depend upon the quality of the teacher -- since Pilates got popular there are basically six-week wonder Pilates teachers out there, as well as highly knowledgeable ones who can really help you to improve your ballroom skills.