This could possibly be the last of the "fighting" MK games. It sounds like Ed Boon wants to go other ways.

Anyway, MK 7 will have returning characters no doubt.
Scorpian
Sub-Zero
Raiden

However the rest are somewhat of a mystery. I think the following will return:
Kabal
Mavado
Kira
Katana
Shao Kahn
Soyna
Jax
Reptile

As for the boss character, well, it may be the Dragon King yet again, but I think with Scorpian's ending it is quite possible he will not. (From Deception) I think Shang Tsung or Quan Chi will return to claim the boss title. Some kind of story line most likely will explain why.
Kabal will have Kira by his side as Mavado returns to finish them off. Kano I don't think will make an apperence here. Kira may be related to Kano somehow. That is why she was in Decpetion. Her storyline will be opened some more next game. Kobra won't return. He seems too reckless to survive into another game.
Soyna and Jax will most likely return to help fight off evil yet again.
Katana will most likely return, but not sure to what storyline she will follow.
I don't think Lui Kang returns, but his ending from Deception is odd. So, I give him a maybe.
Shao Kahn I think as a playable charcter finally will be a return. He needs to reclaim his rightful throne. I think he'll be a fun addition to the characters.
Reptile will return somehow. In the MK world, anything is possible.
Raiden seems to hang around in every game. Another is most likely. He will appear to stop at nothing to keep Earth safe. Even if he has to fight what were his friends once. I think he has a big role to play.
Scorpian's Deception ending looks to be the most satisfying from all of the characters in that game. I hope it does turn out to be true. I don't much care for the Dragon King. I do think Scorpian has a big role to play himself in the next game.
Sub-Zero, who knows what will be happening with him. To me he has gotten boring. The last two games he is more or less a side character.

There you have it. As for the rest of the characters that weren't here, I have no ideas for them. It is a mystery for now until rumors become reality.