Theoretically speaking, his strongest trait is not defense but actually his attack. He is dying because of his sickness but managed to almost destroy Gaara's absolute defense and kill him.
If anything I'd like to make his skill change into this :Sawarabi no Mai [prompt] (20 chakra) Change the field into bone barrier, while the barrier is active Kimimaro and his team can block 1st standard attack, trigger once per round. Active again (40 chakra) to cause high float, can combo even super armor.