"I believe we must all begin to consider a future where the Republic and Jedi Order as we know it are unable to survive the coming days to be a far more likely one," Tholme said, his voice cold. "From what I understand we have been operating under the assumption that both entities were salvageable, however it is now clear that alternate solutions must be considered."
"In order to ensure our security and continuing stability, the Council of Neutral Systems shall be reorganised into the First Galactic Empire. For a safe and secure society."
Thrawn is Conquest, Grievous is War and The Silencer is Death so whoever it is would be Famine. As has been mentioned, it could either be Borvo (for the comedy version) or Unrestrained Ciaran (for the horror version).
Another interpretation of The Third Horseman is Corruption, as the rider of the black horse isn't shown killing crops, but instead hiking up the price of essential foods (and is ordered not to touch the oil or wine), while holding the scales of justice. Considering Ciaran is subverting the law and institutions of the Republic for her own means, even for a good cause, makes her the candidate for the position.