Well, I'd like to throw the Sith Inquisitor and the Jedi Consular in there. Both have their own (rather different) armies and supporters.
Also, Queen Talia of Onderon and Queen Amidala of Naboo might be interesting.
-The Infernal One/Sao, with his powerbase from the Operation (Maybe we could put him against Kephess and his army from the Operation)
-Lord Hoth and his Army of Light
-Empress Teta and her Armies and Fleets
-Cole Centarus leading CorSec
-Mentor + Directive 7 droids
-The Exile with all her Forces from the Battle against Nihilus (Telos Security, Republic Fleet, Companions, Mandalorian strike force)
Also, as comic relief: Sandpeople vs. Jawas (with droids)
I also got the idea to simply use several Flashpoints (for example, Darth Ikorel and his forces, Prophet of Vodal and his forces, Hammer Station etc.). But maybe I should start my own flashpoint vs. flashpoint series for this instead.
Lots of nice suggestions there. And yes flashpoint vs flashpoint does sound very fun. You could do something like 'Hammer Station vs Mandalorian Raiders' and then pit the enemies of each flashpoint against one another - and all the resources seen in the flashpoint. (it would also give me an opptunity to stop pretending to be impartial and go guns blazing into a debate
) You could also end it with an Operation battle.
Just tell me if your going to do it or not - so I don't tread on your toes.