I like Chains of Mephistopheles against PO, since they usually don’t have enchantment removal against DPS. Chains shuts down much of their deck. You may need Karakas or Liliana against BW PO because they bring in Kambal.
Against Thalia, I like creatures. Phyrexian Obliterator, for his immunity to Ballista, as the finisher, and a collection of variable underlings such as Pack Rat, Black Knight, and Plague Spitter. This usually catches them off guard.
I haven’t played against the new card , Lavinia, but I would probably cut Dark Petition and go back to Burning Wish.