Krav Maga is in several ways similar to other martial arts, yet markedly different in certain aspects. Most noticeably, for myself, is the philosophical notion of “never again”, which for the Jewish people is a most sacred maxim, post Holocaust. As well, the conceptual theme of Krav Maga being: “fight with surprise, fierceness, skill and purpose until you are safe”, makes it unique from everything else, as the curricula from various worldwide Krav Maga organizations (I am a high-ranking member of I.K.I.) will all attest.
Still, you can seek out supplementary training that can decidedly enhance your skill sets. One obvious example is Brazilian JiuJitsu, and with proper instruction you can learn basic to advanced techniques of certain submission holds, chokes, skeletal breaks, etc., which can be very useful, especially from a DEFENSIVE perspective. Also, the sublime aspects of flowing with your ground fighting will bode well in a stressful attack, especially with a bigger and more powerful person trying to control you…getting over the psychological fear of being “suffocated” by a bigger body is very necessary to maintaining your “fight composure”.
However, there can be a downside to this amazing art, and that is the competitive/sport side. I have seen very good BJJ tournament competitors become oblivious to the Krav Maga counter-attacking skills of someone well versed: eye gouges, ear-pops, throat and groin attacks, etc. all fair game in a real fight; yet, if your “sport mentality” makes you forget all of this, then it may not bode well in a life-threatening situation, as there are no “tap-outs” in the street.
So, what is the answer? Very simply, it is BALANCE and understanding and training the differences. One of my elite Krav Maga training partners (a veteran of Krav Maga for decades) will absolutely not train in sport JiuJitsu…as his “ground mindset” is completely geared towards inflicting maximum pain on his attacker, whether it be his fist, open hand, knife or gun. He’s a soldier (55 years, now), and I get it.
Another avenue of additional training is MMA, as long as your instructor understands the differences of real fighting versus athletic competition. The timing and offensive/defensive skills necessary for Muay Thai, wrestling, etc., can be most valuable additions to your straight-ahead Krav approach. You cannot help but become a better and well-rounded fighter, especially on the conceptual level.
As well, Jeet Kune Do, TaiJustu, Taekwondo and other martial arts (even the art of the sword) can definitely add to your complete skill set…however, I caution you, develop your basic Krav Maga until it is instinctive and skillful, then, slowly add complementary aspects, as they might fit your own unique approach.