Surely, some of us should have suggested D-league before. However, I hope this time will go into further details.
Same as current league formation, such team could assign three players into D-league. Instead of 5-aside, D-league is 3-aside: C, F and G. We can train the player similar to current training, single-position training or multi-position training, the training minutes are 48+ for full training. Once they were assigned into D-league for that week, these 3 players cannot play current league games/cup games during that week.
The key-point is: players in D-league could be trained at faster speed for young players, so it's our decision whether training in D-league first or call them up to our current league games/cup games.
For simplicity / In order to avoid any abuse of such system, the call-up/assignment should be on weekly basis, just like training results every Friday, every manager has to update the roster list, who will be in current roster next week and who 3 players will be assigned into D-league next week.
For BOT or those who did not assign 3-players into D-league, they will miss such extra training minutes for young players, no adverse impact at all.
Feel free to give any comment / feedback for this suggestion.
HAHA