The way you think about it is not like you are trying to improve your martial art but you are trying to collect styles. This style and that one and that one over there and the more the better and the more well rounded I'll be. Wrong! All styles can be taught many different ways and for many different reasons. Sports martial art is nothing like self defense martial art. No style guarantees what kind of training you will receive but a teacher can. Not everything fits together. You have invested seven years into your training and should be able to know what goes with this unless you have been in kids classes and then you probably would not be able to tell and should spend a few years in adult classes before seeking more knowledge with a different teacher. It is unwise to just randomly pick something. A little candy is good, too much will make you sick. One martial art will make you progress, too much will cause confusion. What clashes, you will not retain and is therefore a waste of time and money you could have spent improving what you already have.
If you have trained in judo for 7 years you should have been doing some striking. Some judo dojo only teach judo as a sport. They have remove the striking. But judo should include atemi waza (striking techniques).
You should not choose a martial art. You should always choose an instructor. Some instructor are better than other. Some teach things that others don't. When I began to learn judo our focus was on self defense. We learned striking and weapons defense from the beginning. We were taught how to defend against someone striking at us and using their momentum the throw them or get a wrist lock, etc. Later I trained in another judo dojo they only taught judo as a sport. I knew this before I joined, but he was the the best instructor that I could find or at least the best for me. There were some other good instructors that I met but I wasn't able to train with them for a variety of reasons.
Personally I would have to consider Muay Thai as a complimentary system for Judo. I say this because Judo requires close proximity to your opponent to be effective at applying your techniques. While Muay Thai does train for ranged striking, it also trains in close quarters combat strikes (knees, elbows, add a headbutt maybe) which will work if you are clinched up and can not immediately execute a throw or establish control.
A traditional form of Karate like Shotokan would also be practical because of the origins of both systems. Most originating countries where systems were developed looked to design system to work in conjunction with or counter existing systems. I know from personal training and experience that Jujutsu (Judo+) and Karate are quite complimentary and actually work for self-defense application.
I'd say boxing if you do strikes or atleast strike defense in your judo. I'd say mauy Thai or mma training if not. I miX judo and boxing it goes well for me.
Muay thai, boxing, kickboxing, sanshoo and things like that generally good.
Karate, Jujitsu as they are both Japanese and work best together.
muay thai or kickboxing
I have learned judo for seven years and want to continue, but I also want to learn a second Martial art, what is a good one to learn in addition to judo? Preferably one that is strike based.
