I'm still thinking the crossbow mission for Bronn is a Cersei plot, but not in the way Bronn expects. Qyburn's offhand comment about the girl with the pox and giving Bronn a bow to kill Tyrion with. I'm still thinking it's a trick. Cersei I don't think is that delusional to think Bronn would actually carry out the assassination mission. Qyburn put some virus/disease in the bow and knows Bronn will bring it north and knows that Tyrion/Jamie will let him hang around. I'm guessing it will be a disease that only affects Dothraki or people from Essos or whatever.
It makes sense. Cersei's goal is to win no matter what. In her mind, the North and the Dead fight it out. By all accounts, it will be a bad battle.
1) If the dead win, then her army buys her time. I'm not sure Cersei 100% believes they could win, but who knows?
2) If the North wins, then she has a foe she knows. Dany's army will have a few main components: The North, the Unsullied, the Dothraki, and the dragons.
For option #2, she knows she has to do something to remove pieces from the chess board. Qyburn's thing is dark shit/diseases/etc. It would totally be in line for him to know of some disease that would impact one population, but not others. The crossbow becomes the smallpox blanket of Game of Thrones. If the show does this, then it would be clever. If the Army of the Dead wins, then the virus means nothing. But if the living win, then the virus can now decimate Dany's army, giving the Golden Company a significant advantage on the battlefield.