For the Steelers I'd say Najee Harris.

Year 3- he needs to perform to start talking about contract extensions and getting real money next off-season. He's currently healthy (hopefully he stays that way as he got injured in campast year).

Most importantly, offensive line. The Steelers had the worst overall offensive line in the league his rookie year, near the bottom of the league in BOTH Pass Block and Run Block Win Rates. Last year they improved to roughly average. This year, they added Suemalo and a lot of their young guys are developing and getting familiar. Either Broderick Jones will start at LT or Moore will have improved enough to keep his job (at least to start the season).

Also decisions like bringing back Gentry, drafting Darnell Washington, and keeping Conner Heyward seems to indicate they are moving towards heavier packages on offense. Replacing Claypool with Allen Robinson is hopefully going to lead to better blocking at the point of attack. I think they really want to be a run-first team and try to take the pressure off of Pickett.

You can make arguments for almost the whole team. The whole offense is really young, mostly still on rookie deals outside of the offensive line and a couple of WR's.