I enjoy recruiting and developing teams more than any other part of this game. Two suggestions:
1 - Make progression tied to some level of (a) playing time, (b) success during said playing time, (c) Coaching skill (fix the flawed system currently), (d) maturity...
2 - Add a maturity rating to players that is impacted by promises during recruiting, success of team, and individual/playing time/maturity of those in front on the depth chart. Say you have a stud immature WR 1st on the depth chart, he would negatively impact game performance and progression of the young guys. This adds a component of quality player and makes having SR laden teams a benefit over all FR/SO teams.
Also, allow the user to choose the Impact player; the Impact player will be the designated leader and his maturity and skill will have the greatest impact on the team. Add a little strategy to who the team Captains are.