By guaranteeing 3 years at most in the relegation league, having similar salary caps, and allowing those teams in the relegation league get the top-tier draft pick so it becomes sort of a high-value development league.
And again, it's obviously a pie in the sky plan. There's no way the NBA owners would agree to anything that resembles relegation for the exact reason that some of the teams would lose their value. It would just be nice if the stakes were higher from a competitive standpoint and you take away all incentive to tank.