You bring up a valid point, few games allow you to be truly "evil".
KOTOR and KOTOR2 (I hope Team Gizka's mod gets done soon, those restored alt. endings mmmmm.)
Did give you a "ok" feeling of the Darkside. But more subtle, yes you only had a single light and a single dark ending. But lots of details leading up to that was colored by some of your choices. Even more so in KOTOR2.
And the darkside ending in KOTOR was one of the more rewarding ones (multiple scenes in the ending lasting several minutes)
And lest not forget Jedi Knight: Jedi Academy where you slay your "friend" then head off for a Jedi and Sith killing rampage. (I littered the place with bodies

)
Considering the fact that the developers actually pitched KOTOR3 as well as The Force Unleashed (and Lucas decided on what we get now) I'm pretty sure the devs will draw inspiration from KOTOR1 and KOTOR2, it is a spiritual sequel in some ways I guess.
But do not expect a alternate darkside path. To avoid making "two-games in-one" the actual lightside/darkside tipping point will be later, possibly the last chapter in the games story.
Also remember that the player character is rather grey, and I'm guessing that throughout the game he "awakens" and gain some morality rather than being just a "machine" and eventually decide whether to follow the light or the dark.
But a game where you can be truly evil through and through...I doubt it, especially anything official (direct or approved) from Lucas film/arts/etc.
I do have a personal project (not star wars related), a game that does not try to steer your characters morality, YOU are the main character's morality. The inhabitants of the game world will react obviously and some things will have consequences, but you as the player is basically deciding how good/evil you should be. It's a text adventure interface game so no fancy graphics. If it get popular though, who knows...