You always have to find a balance between reality and fun. That's the mix BB is trying to achieve. The more realistic possible and most enjoyable also.
1. Skill drops after big injurys.
Like somebody else said, if it is temporal it could make it interesting. Though, having an injury is something "unexpected" and it is also modulated by your doctor level so is a good point I think. You need to make a managerial choice (doctor level adn his salary) and also consider the randomess of the injury.
I don't really see how this feature can add value to the actual system.
2. Hidden potential
Totally unpredictable potentials would be a game killling. Something you planned and invest with a lot of effort and time suddenly becomes unsefull. That's not bearing with uncertanty, it's a bullet on your chest.
An alterbative could be a know range of potential. When he is 18 he may have, say, 3 potentials. You know you he is between 2 and 4 for example. Then, every season when he get's old, the range is reduced in one level. For example, you discover he is between 3 to 4. So, every player at the age of 20 would have a "predictable" potential. This will lead you to take some risk when buying and I think will produce some interesting effects on the market.
3. Skill boost
Interesting somehow. But it is a really complicated to implement. ¿How to decide wich are triggers for the blossom? ¿How much boost? To many undecid things, I prefer to work on some other features first.
4. Ricky Rubio's styles pros
This is something easy to implement and it could be really interesting. This would be very interesting because the strong incentive to train primary skill will be reduced (you can do it later with few lost on training speed) and it will help a lot to create more balanced players. Because, for me is clear that nobody will train a C of pure IS, ID and RB of high potential who was drafted at 16-17. You will have to pay it a salary of 800k every week to keep it.