I don't think that any of the "parent" characters (aka Johnny, Sonya, Jax, Kenshi) will be back, or at least in the base game. Of course they can bring anyone back in non-canon DLC, or just as a story mode cameo.
It would be really weird to create a story about those characters passing the torch to their children (this was the entirety of MKX), only to make ANOTHER story doing the exact same thing. A lot of people actually felt that it was weird that they were trying to give new characters the spotlight, but gave the parents just as much. It just felt odd. Plus those characters have been in the past few games so they can take a break for one game.
Redesign all of the Kombat Kids (both visually and moveset-wise), make then inherit a couple of their parents moves (Takeda finally gets the hang of telekinesis, Cassie can fully control green energy, etc), and call it a day. There will still be tons of classic MK characters so I don't think it would be that big of a deal if just a few take a break.
Cassie, Jacqui, Takeda, Kung Jin, Kotal Kahn, D'Vorah, and maybe Erron will probably all be back considering the story. Kano's son and probably another new character or two will show up. I'm assuming there's going to be 28 characters this time (like INJ2), so that leaves 18-20 classic MK characters. That's plenty.
I already created a thread about this topic a few months ago, but I'll dump my list here to (maybe with a few changes also):
The obvious:
Characters that will be in, no doubt.
- Scorpion
- Sub-Zero
- Raiden
- Liu Kang
- Kitana
The 'most likely':
Characters that aren't guaranteed but will probably be in for story or popularity reasons.
- Cassie Cage
- Jacqui Briggs
- Takeda
- Kung Jin
- Kotal Kahn
- D'Vorah
- Noob Saibot
- Smoke
- Jade
- Fujin
- Kano's son
- Onaga (redesigned heavily)
- Another new character or two.
The '50/50's:
Characters where you can see reasons why they could make it, but aren't really popular and/or needed.
- Erron Black
- Tanya
- Bo' Rai Cho
- Li Mei
- Havik
- Rain
- Frost
- Reptile (need your local jobber to get beat up in story fights)
- Shang Tsung (will probably kill Ermac and steal his souls, like that one MKX arcade ending)
- Daegon (I think they need to start introducing the Red Dragon, probably for Takeda's story arc)
The 'probably DLC':
Just for fun, here are 12 characters I think have high chances of being DLC:
- Shao Kahn (pre-order bonus, 90% sure he will be in)
- 1 of the parent characters I mentioned above (Johnny, Sonya, Kenshi, or Jax. I guess Kano also?)
- Kabal
- Sindel
- Mileena
- Sareena
- Havik
- Reiko
- Pennywise
- Micheal Myers
- Jigsaw
- Completely unexpected guest, like always.
NRS seems to want a roster that doesn't contain all of the wanted character right off the bat, because they like saving some for DLC or for a future game. I think this list has a balance between characters people really want (Fujin, Noob, Smoke, Jade) and characters that aren't really THAT popular if at all (Bo', Tanya, Frost, Daegon). I also think that it also leaves plenty of characters (both popular and not) to do later.