Open Question: Please help me choose a martial art.?

I’m a 19 year old male and 5’7″ (so not very tall). I have been doing Shotokan Karate at a London club for almost 9 years and have reached 2nd Dan Black. Karate is an important part of my life and has helped me with a bully when I was in high school but now I’m finding the classes repetitive and slightly boring.
I would like to start learning a new martial art to further develop my fighting skills and for a new experience.
Could you please give me an idea which you believe might be the best for me to start and if you believe that my time doing karate might make it more difficult to start a new martial art.
Thank you.