Let's do two categories:
You get 1 vote for each category. We'll do it by write-in.
THE IMPERIAL TIE FIGHTER - the lack of Imperial vehicles is obvious and since the Rebels have an X-wing and The Falcon, a stolen Speederbike, and a TaunTaun - I think some outerspace fire-power is just what the Empire needs in the Galactic Heroes line. It could include the never-made-before TIE Pilot. Remember there are a lot of good ideas out there, but you only get one vote!
For the Prequels:
THE REPUBLIC GUNSHIP - the prequel Galactic Heroes figures can almost all be used in some capacity for the Geonosis Arena: the Reek is obvious, but Mace, Yoda, Kit Fisto, Obi-Wan Kenobi, Anakin Skywalker, Padme Amidala, R2D2 Flying, the Clone Trooper, the Super Battle Droid, regular Battle Droid, Geonosian Warrior, Count Dooku, and Jango Fett all belong there. The Separatists will need some more firepower after this, since the Republic has the Naboo N-1 Starfighter, but I can't think of a better vehicle to interact with these figures. The Gunship should come with the never-made-before Destroyer Droid, Poggle the Lesser, and a Clone Pilot.