I've been following this cube for a while and this is a personal opinion- I am not a fan of the addition of more planeswalkers. Mordy, Tamiyo, and Teferi represent 3 more planswalkers for a total of 10.
Planeswalkers are challenging to interact with on the board and this cube is relatively poor in creatures.
They generate value the moment they come down so bounce is poor against them.
They have to be dealt with on the stack, ideally, which this cube can do.
They stand alone rather than slot cleanly into specific archetypes. Instants and sorceries are more synergistic with this cube's themes.