OK. Idea here:
We know that each potential has a hard cap and a soft cap that can be maxed upon training. My proposition is to encourage people to train their own players by... If a player gets traded, their potential cap is automatically set to the lowest end of the soft cap.
So, you run the risk when trading players of potentially making them worse, or of possibly doing nothing at all, however if you train them yourself, this is a non-factor. As is now a star player caps at somewhere between 18 and 20. If you train yourself maybe he caps at the 18, maybe the 20. But with this idea, if you trade them, no matter what they'll cap at around the 18.
24-26 for SS caliber players, etc. you know the drill.
I know you went in with the intention to encourage people to train their own players, but realistically you're instead pretty much putting a serious disincentive to purchasing any player for training. A more reasonable idea to accomplish your stated goal would be to treat players who have never been transferred as having the maximum sublevel - that's an incentive to train a guy you've drafted, which is a nice carrot.