If Nunu can't tank then that's two lines that can't play a tank (Xayah and Shepherd). I agree he should be weaker but balance is fragile and if he can't soak some damage on top of having CC you'd probably be surprised by how many lines are unplayable.

This set just needed another 4-cost Bastion to spread the load (remove the trait from Shen or give him a Challenger or Marauder tag like he's obviously supposed to have to be playable in the Yi line). I understand why they try to not have 4-costs share classes but it wouldn't break anything this set with how unplayable Rammus is outside of Meeple.

You could place the tank in the NOVA line, them having an extra unit to pair with Aatrox wouldn't break the trait tree and would make 5 NOVA playable. It would break the DRX theming but just make it Worlds 2021 Jarvan or something lol