I dont think that would not be bad to have here something in advance, but there should be always any reason why it happened. Cost a lot of time before you develop your team and get somewhere and if something good or bad happens, it should not be based just on luck, because the game could turn into frustrating roulette. Even if it would be rare thing, but more rare it would be, lesser point to implement it has. Also if it would ruin game, even unrealistic parts of it.
1. Skill drops after big injurys.
I can imagine this only if you have doctor worse than lvl3 and that thing would be temporary. Would not be fun to see your player age 28 that he will after injury sucks for the rest of his career. However if he could "recover" and get these skills back after 1 month, it could be taken as a mistake from your medical staff, but would not harm you so strongly.
2. Hidden potential
This is not acceptable. Potential works like that - lower potential player have, less wage he wants, even if have same skills like the one with better potential. You would need an ability to "read" his probably potential by his salary and that would be boring, because only nerds can do that.
3. Skill boost
I can imagine this thing only like something what could happen if you get a player which has any history, that means were in several clubs and is settlet in yours for quite longer time. Something like boost based on his previous experiences. Life is about suprises and sometimes should happen something unexpected. Maybe it would help to us, which are from lower zone, to have someone great finally :D.
And rare 16-17 draft sucks, because it would create an opportunity to have better skilled players for best teams without wage punishment. Also SF would be more easy to train.