Any player over in the US college system right now who is playing D1 or D2 is a huge chance to come back and play NBL especially if they get the right opportunity with their local state championship or SEABL level club to showcase what they have learned.
Note that I have stated 'playing' in D1 or D2, as apposed to warming the bench. But even then, with the right application and desire most of them would be a big chance to get onto an NBL development roster spot.
From there, who knows, it is as much about luck and being in the right place at the right time with the right coach etc... as it is to have to talent to play at NBL level.
More than this, it will be up to the player himself to show how much he wants it, IMHO this is what makes a huge difference for players of the future...