For the upcoming season, Johnson is the best pitcher in the game. His numbers are still out of this world...for K/IP, H/IP, K/BB. All indicators are he'll keep up the same level of performance...until his arm falls off or his knees give out. He's a high reward risk.
It's kind of like trying to trade Barry Bonds. There are no players of nearly equal value...for next season, and perhaps a few more years. Yet you never know...without steroids to speed healing, he might get hurt and start missing time.