I'm assuming Sony and the other companies are in talks about this and something similar was mentioned in the latest PS I love you episode. It needs to be simple and accepted by the whole industry. It needs to encompass the possibility of disorientation or motion sickness so would likely be centred around 'Intensity':

Ia: Fit for everyone. Very low intensity with gradual to slow movement. (The Vanishing Of Ethen Carter)
Ib: Fit for most people but may contain moments of medium to high intensity. (No Man's Sky)
Ic: Fit for people with a high tolerance of movement with medium to high intensity throughout. (RIGS)

Perfect. Yeah, that needs adopting by everyone, or at least something similar that the industry can agree on going forward.

Yeah, seems like a good idea. Hopefully Sony follows suit.

seems like a good idea. it should be noted how "intensity" refers to the actual feel of the game rather than in-game content. so you could have low-intensity M-rated game or a high-intensity E-rated game

It definitely needs to be universal. We're talking health here.