If he works on his 1-on-1 defense ala Shane Battier (constantly breaking down film of opponents, learning their tendencies..etc) focus on his team defense( lately it has been much better) and learning how to post up smaller/weaker guards, I'd say he would at least be as good as Wade in his prime.
I think he can be like Mikal Gordan but with a better 3pt shot and better left hand. James Harden left hand >>>>>>>>> MG left hand
Harden's defence has been better of late, he is so tough to stop because of his offensive set and he is strong physically