Hmm.. since you've already added
Archfiend of Despair, what about adding
Havoc Festival for the combo? with both on the board, every player that isn't able to break this in his next turn dies.
Since you get
Xantcha back, if the player that controls her dies,
Harmless Offering is a good card to get rid of her again, without killing her.
What about
Crown of Doom? It's always fun to play hot potato, especially if it can't come back to you (unless the current controller dies).
Stolen Strategy could be interesting. It's expensive manawise, but it offers a massive card advantage (one card per player and turn) if it doesn't get removed fast.
Besides that, I have to agree with superspenky, mogis isn't a good idea, since the player that currently controlls
xantcha will sacrifice her first.