Oh, hmm, let me think....
Oh! A dashing, roguish villain as a potential LI.
... 
I love it. Especially if he's a biotic.
Don't ask when why that's a prerequisite for me, I can't explain it. I just love biotics.
Alright, but something more though...lessee...
Tbh, I would welcome a playable backstory. It does not have to be hours long. Just 15 minutes, or so. Some kind of life and attachment for the PC outside of this organization. Something with family, friends, a community. Then later you may run into them again. It's a great way to notice how you've really changed over time and adventures. Or notice how they've changed. Perhaps you're even on opposite sides now...of whatever.
Or you're still the best of friends, and it's just that you're able to talk to them in a different way than everybody else. Because they knew you before all this started.
But yeah, a playable prologue backstory with a life and attachments outside of the PathFinders.
I really love it. I've been rooting for the idea of a playable origin story for a while, especially if it involves the character who will become a primary antagonist at a later date.
It would really be cool to have an approval rating with the antagonist. They're opposing you for practical or ideological reasons, but the way they feel about you as their opponent can depend on the tone the interaction takes on early in the story.