I'm mostly posting this to have it written down so I can come back to it when I have the time. This requires an in-game editor.
My idea is that every season reset day, all players that are exactly 18 years old in a given league (excluding foreign-based loanees) would be available to all clubs in a 2 or 3 round "draft" similar to American sports. The first picks in the draft would go the newly-promoted clubs (in England, the 1st pick would go to the promoted Playoff club, with 2 and 3 going to the 2nd and 1st placed clubs respectively). The remaining picks would go to the clubs in o