I always thought a Bloody Flux card would be interesting. It would probably have to be Targ, Targ/Lanni or Neutral. Have some form on burn or crippling effect.

A Fighting Pit may make for an interesting Targ location as well.

I don't know if it would be an event or an attachment, but to go with the Facelessmen Guild theme there could be a Change of Face card. Perhaps attach it to a character, or use it on a character to discard them and replace them with someone else. A bit like the non-banned Jaqen H'ghar.