If the Dark One was around.....

[Thalia]

Mwahahaha! Interesting question.

Soren would for sure end up Forsaken, as a trade off for knowledge. No cost too high.

Oriena would also probably end up on the "dark side", just by her associations and general attitude.

I think the rest would stay with the Light. Though having said that, Thalia could probably go either way, depending on what people she surrounded herself with.

Well, I was wrong about Soren. He's light-fearing. Who'd a thunk it.

Ori, probably still dark side, though I think she'd need incentive. She wouldn't do it for the sake of it.

Thalia ended up Black Ajah in her 3rd Age life. But generally she's a "light" person, it does just depend on her influences.

Did I really only have 3 characters back then?! (edit: oh, nevermind, I said "the rest")

Asha - light
Eidolon - light, for sure
Zeke - well... (darkfriend with delusions of grandeur)
Helena - she's for the light, though you wouldn't know it. Her soul is all about balance so I suppose under the right circumstances.
Morven - could probably go either way. Sekmet was SCARY
Natalie - light, obviously, though she can make very morally grey decisions. 
Nhysa - hmm, you know I'm not sure
Noemi - probably light. Though if Hades had a 3rd Age life, whatever he was
Raffe - he was for the light
Sesuna - light
Sofia - Forsaken material
Tenzin - light
Vena - light
Visha - dark most like, depending on if she was persecuted. Or maybe she'd be unlucky and born shadowspawn 0_o
Zhenya - light

Bastian - Forsaken, but only because the wheel fucked him over y'know? The guy SHOULD have been a hero.
Esper - neither, she's in life for herself, sides are irrelevant
Gideon - though he has an alternate 3rd Age forsaken life, I don't think it's inevitable for his soul. Had he just been born fresh in the 3rd Age, he could have easily been utilised by either side. Left to his own devices though, probably an easy win for the dark
Nesrin - Forsaken life aside, if she was just born straight into the 3rd age, I'm not sure. But I can't see her putting up with that White Tower rubbish.
Lore - probably light, but she'll take the hard path if she believes in it morally, so could easily be corrupted by a Nikolai Brandon brand of Forsaken