This seems like 2 different questions. To me you can't 'prove' something until you've already shown it. I think Rocco has a lot to 'prove'. He's shown that he is really good, he needs to prove that he can take the next step while breaking in new receivers. Likewise, I'd say special teams needs to prove that they can consistently be a strength. I would love to see our RBs build on what they showed last year.
That's a different question than what areas I'm most concerned about. I'm not concerned at all about QB, RB, or even STs. I think a lot of responses have been about what groups need to 'im'prove. To me that is clearly DL. I'm choosing to believe that LB will fix itself through health and depth.