This is part of what seems too arcadey/gamey to me. For me, molding players would be hiring a certain type of coaching staff, setting up practice schedules (team and individual drills) with coaches emphasizing certain areas more than others, and have the CPU determine progression using the abilities of the coaching staff (to teach) and players (to learn). Like it really happens in the NFL.
Scouting, coaching staffs, player progression, expanded rosters (including practice squads) and team financials could be greatly expanded on to make a much more realistic representation of the NFL outside of what happens on the football field.
I don't want to have to force STATS (either in games or in practice) on players for them to progress.
To me, XP progression is just another gimmick. Now, I can understand why others really like it. Just saying that I personally hate it and would rather see something less gamey and much more like how progression happens in the real world.
I would want to be able to hire a different RB coach or a different QB coach to help Stafford's accuracy. A players performance (fumbling) doesn't make them regress. Regression makes their performance worse. Not fumbling doesn't make a player progress better, the player progresses in ball security and then doesn't fumble as much. Same with Stafford's accuracy. The control I want, is to hire a different position coach, and/or change the scheduling of practice drills to emphasize ball handling and QB accuracy and let the CPU simulate the actual progression.