I think you are making this too complicated. Simply keeping it like the draft would be perfectly fine. No need to change anything with the potentially possibility or how a player improves in certain leagues (Especially because in some nations it is much easier to get to and stay in a higher league). All i think needs to happen in order to make things more interesting is to hide the potential with the draft ball system until they are 20-21.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs