I recommend Shaolin Kempo Karate because if you learn that martial art, you can finish some one off with one big blow. Kickboxing won't be worth it because TaeKwonDo (the martial art you learned) is pretty similar to it since tae kwon do has kicking and punching just like Kickboxing.
well, it's a big oxymoron, so i don't know how much to trust it...shaolin...indicates a chinese art, but karate specifically designates it as a japanese art...something i do not understand...any way...go watch a class of each and decide what you like better.
Kickboxing out of those options....
You should look into some kind of grappling to compliment your stand up though...Judo, JuJitzu, Wrestling...whatever. You don't want to get lumped up if it goes to the ground.
I study shaolin techniques and u must be mentally fit as well as physically. Shaolin is not about self defense but self control and inner calm. If u can meditate for long periods and have acrobatic skills then shaolin is the most powerful martial art for body and mind, no one will beat you!