Very few challenging athletic experiences will make a kid a worse soccer player. Some of them will make them much better. The movement ethic seems to translate very directly to the soccer field.
Softball teaches judgement of trajectory.
Volleyball is great for keepers. Since the ball runs true and fast, player learn to collect and turn with the ball. The true roll both makes ball control easier and makes mistakes more obvious. Karate, gymnastics, dance and other activities certainly contribute to a kid's athletic advancement. Any aerobic activity contributes to their fitness level.