Well, the already-existing classes are based on archetypes from the movies and previous expanded universe. The bounty hunter is based off of Boba and Jango Fett, the Inquisitor baed of off Palpatine and Dooku, the smuggler based off Han Solo, etc...
To me, it seems like there aren't many archetypes left from the movies. But! There are some from Old Republic in general that Bioware hasn't used as a class yet, and that's the Mandalorians and the Echani.
The Mandalorians and the Echani are rivals - exact opposites - yet they share values of honor and individual prowess. The Mandos, as we know, are mostly Empire-aligned, and the Echani from KOTOR2 are more partial to the ideals of the Republic. The reemergence of the Echani, and the conflict between the Echani and the Mandolorians, would be a solid ground for an expansion.
They both show skill in close combat, and even unarmed combat, so I'm thinking they would be good melee classes )this would also allow the Mandolorian to be even more different from the bounty hunter class) Here's something along the lines of what I'm thinking, as far as class and subclasses go:
EMPIRE CLASS - Mandalorian Raider
subclass - Mandalorian Gladiator (tank/dps) - uses heavy armor and large melee weapons, like the axe Mandalore uses in KOTOR2.
subclass - Mandalorian Scout (dps/healer) - uses light armor and a vibrosword.
REPUBLIC CLASS - Echani Monk (look the Echani up on Wookiepedia to learn more about them!)
subclass - Echani Sun Guard (tank/dps) the mirror of the Mandolorian Gladiator; uses heavy armor and double-bladed vibroswords.
subclass - Echani Duelist (dps/healer) mirror of the Mandolorian Scout;; uses light armor and fist weapons (since the echani excel in unarmed combat so well)
I feel like the Mandalorian would be too similar to the bounty hunter, although it would be cool and represent a new oppurtunity for story. Personally in the next expansion I'd rather see them do alot more with space ships than add largely unnecessary classes. I love space combat in general, and I think alot of people can agree it's somewhat lacking. The ships are cool and all, but there really isnt a whole lot to do with em that doesnt get repetitive.