No no no...
If you would like to make BB more like NBA when it comes to young players you have to work from the existing system i BB, not the NBA.
We have the draft where 18 and 19 year old players are available. Maybe... we could mix in a few older players in the draft to simulate late bloomers, foreign players and so on. Their skills could be higher than the original draftees but should NOT be as high as a 18y draftee with 3 years training (if he is 21). 21 might be the oldest a player could be drafted and he should NOT take a spot in the U21-squad, he should not be that good.
You could view all 20-23 year old players on the TL that have recieved some training during their first seasons as players from D-league
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Or you could redo the whole BB-training system and really make it complicated, bigger rosters, salary-negotioations, two-waycontracts, D-league team, individual training to some degree, staff hiring for all positions, including D-league... but then it would not be BB, would it?