Many players would agree with me that their isn't enough places to farm some heroes in elite stages so I suggest a new campaign mode.

Expert mode
No hp or energy regen
Drops 2 SS per stage
Cost 3x the stamina of a normal campaign
Unlocks at level 80 (chapter 1 is level 80 and gradually increases as we get to chapter 14)

You can remove some heroes from the current shops that been there a long time and move them to these new expert campaign mode. You can put some heroes that have only 2 stages of farming here also. Maybe place ninja here also.