I see a flaw in the general thinking, the Sith Triumvirate on the one hand and Revan and Malak on the other hand are from different periods so the can't be together.
I was thinking my self about a similar scenario where the Sith Triumvirate faces the Emperor. This would be a real challenge, especially Darth Nihilius can make it very hard at least for the Emperor because he would consume him (which could backfire because the more Darth Nihilius would consume the more hunger he would have and consuming the Emperors power would not only give him a temporary indigestion but also give him a unquenchable hunger, this is very bad for the whole universe), Darth Sion should also not be dismissed because of his grasp on the Force (fighting would be either on Korriban or Dromuund Kaas, either place is steeped in the Dark-side which would aid both Darth Sion and Darth Nihilius). Sarth Traya is just manipulative.
Simple, after Nihilus consumes Emperor Vitiate and attains his unsaitable hunger he continues to eat Sion over and over until he's full since Sion cant die and everytime Nihilus eats him he can just come back to life.
proof of this right here.