On offense...Cam'Ron Harris. Needs to be a steady presence in all facets of the game. Grind out dirty yards in garbage, pass catcher, blocker, salesman, red zone bully. If the offense takes a minute to get right, he needs to be the guy that steadies the ship.
On defense...Quincy Roche. If he solidifies one side as the complete player he showed himself to be at Temple, it will allow Gregory Rousseau to do what he does best and play inside more often. Secondary to this...whoever is replacing Shaq Quarterman. I'm not sure if its going to be Sam Brooks, McCloud, or Jennings. Not only do they need to be as good as Shaq was downhill and between the tackles, but for our defense to get better, they need to get to the edge faster than Shaq could. Couple this with providing 6 sacks like 2018 Shaq did and I think that player turns this defense from good to great. So, tl;dr Quincy Roche and the Mike linebacker.