Choices seem to be very limited:
Asajj: Nighsisters only
Poggle: 30% offense for droids and Geonosians
Dooku: All allies 15% evasion and offense up for 2 turns after evading
Nute: Stealthed allies gain 55% critical chance and gain 50% turn meter for first crit hit
Greivous (Zeta): Enemies have -40% Crit avoid and -50% defense. Turn meter bonus when target locked enemies are hit.
Maybe Zeta Greivous?