I like the skill drop idea, but only if its temporary and not permanent. Just imagine you train a player for a few seasons (=one "real" year) and some random event destroys some of your hard work - how humiliating and de-motivating would that be? If that happened to me I would instaquit this game.
Hidden potential - I dont think this would make sense, maybe if potential in the beginning would just be an estimate and then could raise or drop one or two levels, but completely hidden... I dont like this.
Skill boost - random and thus unfair
Ricky Rubio - I think rookies should become stronger in general (maybe the top-rookies salary range up to 10-15k), but 2 or 3 16 year old top-guns in the whole game? Whats the profit for everybody here?