I have been heavily invested in PO for the past month or so since I put down Blue Moon and with Lavinia I think PO players need to add Red.
This means either cut White or cut Black. If we cut White then Mentor goes and not much else. If we cut Black then the DT, Yawg Will, and Tendrils go.
Let's look at cut White first. Without Mentor do we run Sia or Pyromancer? Probably not, we add 2 Pyroblast main and there are other answers on the board like Sudden Shock, more Pyroblast and Lightning Bolt.
If we cut Black then we still add Pyroblast and Sudden Shock and or Lightning Bolt but you may add Grapeshot main maybe 2 Grapeshots.
The largest issue with losing White is losing Fragmentize and Disenchant, but we still have bounce like Repeal, Chain of Vapor maybe Echoing Truth.
Losing Black I think changes things more but offers the most room to fight the meta-game. Keeping White with maybe Enlightened Tutor or Silence really changes how we play the deck.
I think we really need to consider Brainfreeze as a PO kill spell, I have had great success with Brainfreeze over Tendrils and it is way more Flexible. Using Brainfreeze means we may need one Tormod's Crypt main but that's not a bad thing.
I'd like to some feedback either way from PO players. I'm going to try going with Red and cutting White then cutting Black and then a Blue Red only and see what happens. I do not have a list yet, but when I do I'll post it.