I actually started a thread about this earlier today but canceled after realizing the OP would take entirely too much typing for me to articulate the point. lol However, I will try to sum up my take on this briefly.
First off, initial player ratings and subsequent updates from EA have to begin to be calculated using a standardized universal formula to convert statistical player data, similar to
PFF's or some other entity's,
(I learned about PFF through Callebro's post in another thread) into base numeric ratings and DPP for each player.
Then Madden needs coach/coordinator ratings, an individual body parts wear/tear injury system, player influencing playbooks/schemes and dynamic player potential/OVR depending on the team specifics, all of which were in HC09.
User or CPU player performance/stats should ONLY effect DPP's streaks, tendencies and traits but NOT directly change base numeric player ratings or DPP's consistency and confidence ratings. Base numeric player ratings, along with DPP's consistency and confidence ratings should ONLY be effected by injury and general exposure, with playing time factored in, under coaches, coordinators, playbooks and schemes, which become set over time, to actually change those base ratings.
For example, Wes Welker would have base numerical player ratings and DPP. If in Miami under that coaching staff and system he would have a different range of numerical trainable skill ratings
(catch, route running, etc) along with a different OVR and player potential, than when going to New England. However, those trainable skill ratings ranges are initially only attainable through streaks, with every season under the same coaching staff and scheme, directly changing Wleker's base numeric trainable skill ratings, consistency and confidence, varying in degrees by playing time.
So Welker could increase his base run blocking rating and decline his base route running rating after 2 season's in Miami but still initially be capable of better route running and worse run blocking streaks in NE because of their coaching and scheme.
Voilą! Instant realism, imo.