Aikido is good but I prefer Aikijutsu if you can find a teacher in that. It's a little harder to come by than Aikido. Jo and naginata are fun too. Kenjutsu is also very different from Kendo and a good Kenjutsu school will give you much insight into the culture too which might help you understand what you already have even a bit better.

Some people do Karate with Ken but I don't know enough to pass judgement on that. I guess it really depends a lot on what teachers you have and what is being taught too.

Are you looking for an "unarmed" art or another weapon based art? If unarmed, I imagine that Juijutsu or Judo would be a pretty good match. And Aikido could also work, having similar footwork....

Aikido is a good one or most Japanese jujutsu. I prefer more jodo the 4 foot staff,

Jui jitsu in samurai style and all its derivatives. I think fenicing saber would help also.

As the title says, I'm looking to improve my Kendo through cross-training (other than the usual cross training like improving cardio fitness). I have done Kendo for almost 9 years now, and I feel like my improvement has been plateauing. Because of this, I am interested in picking up another MA to supplement Kendo. I've been doing Iaido for three years now, so please don't recommend this.

I have heard that Aikido is a good pair to Kendo, though I'm interested in hearing what you guys have to say.

Thanks in advance.