Love first three suggestions, not so much the fourth one.
First does that in a way already, in cases where a player is being trained. If a player gets injured, he can't train, so it has the effect as if he lost a few skills.
If two ever gets implemented, it would go hand in hand with the draft. It would have to be more hands on, managers should have more control over it.
Three, have players who are early vs. late bloomers.