Aikido, my Tae Kwon Do teacher's friend came in and demonstrated a few techniques one day and it's great.
Also Tai-jutsu would be one I'd really like to learn. (it's the actual hand to hand component to Ninjutsu)

Considering I already know Shotokan Karate, and therefor Karate too, I'd have to go with Pi Gua. Or, as a tribute to Broce Lee, Jeet Kune Do.

I'd like to get some professional training in Crane Style Kung Fu, it's just wickedly cool stuff :)

Long Fist or Crane.

shaolin fist, praying mantis, jeet kune do
:)

I already have done Judo for 7 years. Now I train Aikido. When I saw that Kano had Aikido too I was quite surprised. But Aikido is hard to put in a game and I didn't see loads of similarities with the real thing.
Since I do Aikido I think it's the most superior martial art. Im not talking about how powerfull or effective it is. I like Martial Arts in common and I will train lots of them. Maybe I start to train Hapkido after a few years too (I thought that up before MkDa). Hapkido has some similarties with Aikido. I also train Aikido with a staff ("Aiki-Jo") and with a sword ("Aiki-Ken")