I feel like adding an effective changeup or curveball to his arsenal would work, but evidently his entire life his coaches have said "Nope, that fast ball is good enough."
And no, I don't think his slider counts as a second pitch. The only reason it was "effective" at any point in his career is because everyones waiting on a 100 mph heater
If he could develop any sink or run on his fastball it would be devastating again. Look at the soft contact that Hendricks got out of his fastball last season, and it was only 86-89 max.
Also, I think he tips his slider and is much easier to lay off than it used to be.