I like this "hold-in" idea. It would really add to the realism for contract negotiations IMO. I could be mistaken with another sports title, but at one point didn't Madden have a "character" rating on each player? Maybe like Madden 25 or around there? 
Regardless, that'd be awesome. Honestly I wish there was more penalty to the team for players being unhappy with their contracts (or any unhappiness at all with contracts). Perhaps locking the players progression until a new contract is received or something, idk.