Allow player to apply for the Draft but retain their eligibility if they don't hire and agent (an unpaid adviser is OK if they're not a professional agent or affiliated with one). If the player is not drafted or doesn't like their draft position, they could return to college.
This helps college programs by causing players to attend classes in their second semester since they may want to return. It also makes players listen and respond more to their college coaches for the same reason.
It's also unilateral and requires no approval from the NBA and doesn't hurt the NBA. And it helps the student-athletes

I don't see a downside, do you.