Question 007: Train in two arts

I read your page about people who think about taking up a second type of martial art as well as the one they are already taking. I am currently a 17 year old Karate student and I'd like to take up another art to further increase my discipline and fitness. I enjoy martial arts more than anything, and so I am emailing to ask which would be a suitable art to take with karate. I was thinking TKD but I'm not sure the two would mix suitably. What do you think?