I've taken multiple martial arts in my life. I think the next martial arts you need to learn is dependent on what your goal is. If you want to fight people and hurt people MMA style you should invest time in Jujitsu or Wrestling. I think that is what you want because you spoke about versatility. If you are taking fighting for just the love of the art form and bettering yourself things like Tai Chi can be very rewarding. Certain fighting styles I've taken like Aikido are almost completely useless in real combat but it all depends on what you want from the fighting style. BTW Prinny, I studied Kendo and definitely wasn't a fan of it.