Now there are a lot of Martial Arts blog posts about cross training (like here and here at My Journey to Blackbelt, and here at Martial Arts And Modern Life). I'm not a fan of training multiple styles of Karate in order to round out training, but I do think that training in different fighting styles would work, like Karate and Judo, or the more obvious, Karate and Kobujitsu. There are also a lot of theories on when you should try cross training, and a lot of sources say wait until you have the basics down (shodan) before trying something different.
I started to take part in judo classes at the Fredericton Judo Club, and the work-out was much harder than what I am used to. The nights I can attend, we typically work on ground work (Jujutsu), but I think I'm going to see if I can get more traditional Judo training.
I chose Judo as a secondary Martial Art mostly due to my lack of training in dealing with an opponent who has been able to break through my defense, or has managed to tackle me to the ground (the opposite of Masami Tsuruoka, who was accomplished in Judo, and then practiced Karate to defend against a larger opponent). I'm just starting with Judo, but I am REALLY excited learning a lot of new things, and making even more friends who are dedicated to their Martial Art.