Just an idea.
I'm personally not fond of the D-League idea as it stands because we don't need *more* trained players - there are tons and tons of players being churned out as it is, and it should be a sacrifice to create a special type of player, no matter how annoying that can be. But I think looking at a D-League as a true developmental league might give another angle, so with that said, here's the idea:
1. eligibility - any player you've drafted, or players from your nationality that you've signed, with a maximum potential of all-star.
2. training - will be somewhere between half to 1/3 the speed of one position training (with a level 4 trainer). However, while on the D-League team, the player will be considered to have allstar* potential, so any draftee can theoretically be made into a useful player but not a superstar.
3. salaries will still be paid plus some sort of fee per player for the training, maybe also an additional flat fee for the additional "coach".
I think the crux of my suggestion here is that it shouldn't be used to create even more supertalented players, nor should it be even close to as effective as training players on a real roster, but it should definitely be possible to make guys who are otherwise trash into players who can legitimately belong to your team, if their starting skills are fine.