Given what happened to Caesar and the previous two triumvirates I seriously doubt Brutus and Cassius would dare go that far. I could easily see them being Consuls and wielding a lot of power, but while acting publicly as just leading members of the Senate. Marius, Sulla, Pompey, and Caesar; they would see a pattern that whenever any man tried to claim extraordinary power bad things happened. Being no more than the leaders of the Senate was about as much as they could safely hope for.