I'm going to post what I said in the other thread talking about Goro DLC.
I believe that multiple people from NRS have said that they won't make us pay for something that is already on disk... thus.. we won't have to pay to "unlock" content.
1) This leads me to believe that when the retro costumes and fatalities are released, they will be free.
2) If bosses are released, they (Goro, Kintaro, and Shao Kahn) will be free.
3) If Brutalities are already in the game but locked, they will be free to unlock.
*The only exception to this would be Skarlett. I know that somebody found her character model hidden in their game, but she has a model because she's in the background and in the story. She doesn't have a moveset or anything yet. If she does, and If NRS keeps what they've said, she, too, will be free.
I really hope that NRS sticks to what they have said throughout development.