I expect to see Mac take a leap this year. He's shown us flashes of his upside/ceiling, but hasn't consistently put it together. He went through some adversity last year with his injury, his receivers' injuries, the disarray around him, and the crazy L's that should have been W's. Having survived that, together with the apparent team & coaching improvements around him, he should be a much improved player this year.
The part of his game I'll be looking at most is his mobility (his arm-smarts-leadership are all very good). At times he has moved up in the pocket well, scrambled well, run well, thrown on the run well, reacted to pressure well - last year's Ravens game in particular. At other times he has looked underwhelming and panicky in those situations, and didn't see lanes that were there. He's got to become more fluid and relaxed when the pocket starts to break down, break through contact like Brady learned to do, and use his speed to get outside.