Don't recall that fight ever getting made in the past. As you said though, it wouldn't be a bad fight. I think Hearn will try and keep Zhang undefeated though until cashing him out so I don't expect him to fight anyone too dangerous.
I think Hearn would love to make AJ v Zhang down the line in China. Mismatch, but one that brings in the £s if Zhang's record is protected till then.