Before the Audiences release, you were able to target based on new vs returning visitors. I know the previous implementation was not ideal, but I would really love to see this general functionality come back in a more robust way.
Please add this to your list Optimizely!
Thanks for the feedback! We've brought this condition back, with the new name "New/Returning Session." The new one works slightly differently than the old. When a new user visits, they'll stay "new" for the duration of the browser session, i.e. until they close the tab. Hope it helps!