Im learning BJJ at the moment and have some boxing experience but I think learning so Muay thai would be good. What about Judo?Wrestling?
For standup, I'd train heavily with Muay Thai.
For groundwork, I'd train mainly with no-gi Brazilian JiuJitsu with a focus on MMA application that includes strikes (versus sports JiuJitsu). Sports JiuJitsu, like what is seen in the Pan Ams and Worlds, is great if you look at a fight as taking place solely on the ground and with no use of strikes. Learn the basics of BJJ (up until about 4-stripe Blue or 1-stripe purple) and then concentrate more on no-gi JiuJitsu and make sure your instructor incorporates both strikes and uses guards that are good for evading strikes (Lister Guard, Penn's Damn Good Guard etc).
Within your groundwork, I would incorporate Wrestling in order to learn how to better control your opponent once on the ground and also to learn to have proper takedown defense.
Spend some time with a Judo sho-dan in order to pick up excellent hip control and throwing power, which can allow for great transitions into your ground game.
So, in order, to maximize your potential stick with Muay Thai and no-gi JiuJitsu with MMA application. The combination of these two will allow you to have solid standup and ground game. Polish both disciplines by incorporating Judo and Wrestling.
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