If you just want a clean, straightforward one-turn kill, Chandra's Ignition is a pretty good option, able to smite every creature and player in such dramatic fashion that they probably won't really mind
Simic is a fun guild but tends to have ruther clumsy cards that are hard to place in many archetypes
Haste and toxic 3 are good starts on getting everyone corrupted
Its better in some cases, but having to pay mana will sometimes be a hurdle
Even if they do have an answer, Codies deep interaction suite means its more than capable of winning a counter war
The safe bet would be Ikoria given that she may feel she has unfinished business there, but she could appear anywhere that the preservation of nature and the development of civilization are at odds, whether thats a new plane or an old favorite