Definitely not something that can just be added in.
The one thing I dislike most about the XP system is that ratings only move in one direction until they hit a certain age. A 21-year old can be upgraded to a 90 OVR player by age 22. He then can ride the bench for 5 seasons and hit the field at age 27 at 90 OVR. No progression or regression, didn't earn any XP 5th on the depth chart so he's the identical player that played 5 years prior. Would love if a player had 'rust' over not playing for an extended time.
I feel with the XP system it's going to be stuck this way. People wouldn't be happy if they spent a season upgrading a skill just to see it reset the next year. For older players and regression it's expected, but if your 24-year old RB was upgraded from 88->93 BCV and come next year he's down to 87... plenty of people wouldn't like that.
One thing I would like to see is more external factors effecting a players OVR. I think there still is a production rating that impacts it (maybe?) and confidence has some impact if it's high or low enough. If the game could add more of these factors, such as chemistry, hot/cold streaks (return), etc., it would help players vary week to week and year to year.
Ultimately I would like to see XP removed in coach/owner mode for players, replaced by training/coaching influence. Let you choose what you want a player to work it, maybe your coaching staff and players learning ability effect how much their rating can increase.
Leave XP for Player mode and upgrading coaches. When playing as player you can choose what you want to work on, you just can't tell/force the other players to learn what you want them to.