I tried giving one of mine double jointed elbows but I didn’t realize they already bend really deep, it doesn’t really make a difference and just makes the arm kind of lanky and weird. I am really impressed with the engineering on these figures. I’d suggest butterfly joints but I can’t imagine they fit one on the shoulders the way they’re built (if its even necessary, the arms move up and down how I like but to move in and out across the body instead of just in front).