These characters were either playable, or were fightable opponents in the segment of story I played, which incidentally focused on Johnny Cage.
Johnny Cage (Playable)
Jax (Fought against)
Shinnok (Fought against)
Johnny, like most of the returning cast, has held on to most of the classic moves people will remember him by, including his Shadow Kick and Shadow Uppercut. Jax is a fan-favorite who hasn't been seen in any way before so his appearance as a combatant is a pleasant surprise. At least at this point in the story, he seems to have been possessed by the other dude I fought, long-time MK big-bad, Shinnock. He was the final battle of Johnny's chapter, and he seemed to have Shang-Tsung's move-set, with fireballs that come up from the bottom of the screen.
These characters made appearances during cinematics for the very first time:
Smoke
Nightwolf
Kenshi (confirmed playable in Mortal Kombat X mobile)
Fujin
Stryker
Kabal
Other than Kenshi, who was revealed as being playable in the mobile version of MK X a few days ago, none of these characters have been seen in any previous trailer. Are they playable, or are they just chilling in the background? We have no idea yet, but there are a bunch more slots on the character select screen that need to be filled so it's quite likely most if not all of these characters are indeed playable.
One more we know is playable is Sonya Blade, Cassie Cage's mother, who was officially revealed in the new "Cage Family" trailer released this morning.