Hmz...just a thought (assuming there will be at least 3 talent trees)
Tank: Jedi, Republic Commando, *Sith class* , Sith (since troopers are know to use blades in close-combat, with the right talents they could be tanks).
DPS: Jedi, Smuggler, Sith, Bounty Hunter again, with the right specs.
Healing: Republic Commando (medic tree), Jedi, Sith, *Sith class*
Ranged: Republic Trooper, Smuggler, Bounty Hunter, *Sith class*
AoE: Republic Trooper, Smuggler, Bounty Hunter, *Sith class*
I think this looks pretty balanced.
If I look at the gaps...a Tank, healer, ranged, aoe....I think it could very well be a Hunter-Killer droid.
We know there are medic droids, medic could be a talent tree. Their strong armor could be used for tanking (which would mean it could be a melee droid as well). And AoE and Ranged speaks for itself.
And even if you take out the healing, that could be done by a Bounty Hunter (thinking of illegal stims and such here) it could be possible.