Kali/Escrima. Its one of the few arts that still remains true to its original combative history. You learn weapons simultaneously with empty hands. I've been doing martial arts for over 24 years and have yet to find a style that's more effective in real situations on a regular basis. Were I to choose a single art to study, it would be Kali/Escrima without hesitation.
Look at it this way. How many other styles will have instructors with knife scars from actual death matches?
Kickboxing is great for basics, which is what the majority of fighting actually is. Any grappling (Sombo, JiuJitsu, Wrestling) art will be mediocre at best against multiple attackers. A traditional art (Hwa Rang Do, Shaolin, Tai Chi) won't really compliment your kickboxing training very well. Pressure point fighting (Ryukyu Kempo, etc.) is good, if you have a couple decades to dedicate to it. Krav Maga is good but its the method of training, not the techniques. Its very basic but trained at a high intensity. Like I said, I would suggest Kali/Escrima, but make sure you find a credible instructor and not someone that's just arbitrarily swinging a stick around.
I saw a couple of articles years back about the "dog brothers" and some real pics about their training with Kali/Escrima impressive stuff indeed, but also nuts, it all depends how bad you want it!