Humans has historically employed four-drops in the sideboard to give the deck a little bit of post-sideboard staying power, and I like what Urza and Yawgmoth bring to the table for this purpose
The only reason it doesn't sit higher on the list is that milling in Commander is still quite hit-and-miss , so its role in the format is likely to be minimal
9: Each opponent chooses a permanent they control of each permanent type and sacrifices the rest
My next addition might look similar
Try Coercion, Dark Inquiry, Diplomacy of the Wastes, Doomfall, Memory Leak, Memory Theft, Never Happened, Perish the Thought, Reckoner Shakedown, or Toil of the Invasion, if you just want to pluck a card of your choice out of an opponents hand for three mana