I think the obvious answer isn't in your list. Tutu.
His rookie year was a total nothing. Almost no offensive snaps with no targets and few and poor STs play. His first catch will make him the most improved Ram. If he catches a TD, he will be far in the lead for this made up award. If he joins the actually rotation and fills the deep threat Jackson role he will be light years ahead of anyone else.
If you start off as a total wash out and then become a player you have really improved. Many doubt his ability to even make the team. We've gone through that before. Without Tutu, I agree that Garrett might surprise. He also didn't play a lick so any success for him would be a massive improvement.