I think no-one needs to explain it but Yuumi is an untargetable Sona in late game.
That said, Yuumi ends up being a problem for people in the late game.
So in my opinion, after Worlds, you can use this time to adjust Yuumi to be more... item based?
Hear me out.
Nerf the damage of Yuumi's Q more, but revert most of the slowing power it had or increase the slow duration. Aka, focus on the slow rather than the damage.
Let W give a percentage of Armor and Magic Resistance as well. Move item benefits Yuumi has should go to the one she's on. This is already the case with Bami / Sunfire cape and other aura abilites, but being able to give your ADC your own Guardian Angel or Zhonya's makes Yuumi into an item threat and not a heal threat.
With this, flatten the healing on E but allow it to scale with more than just AP.
W gives Yuumi an *adaptive* bonus, yet there are no items or abilites Yuumi can use had Yuumi made AD.