So, Ed Boon says characters from each era are in this game. From the leaked roster of confirmed characters, there's representation from the first MK through Deception. Missing are new characters from Armageddon (Taven, Sareena, Blaze and Daegon), MKX (D'vorrah, Kotal, Ferra, and the Kombat Kids), and MK11 (Geras, Cetrion, Kollector, and Kronika).
Taven and Daegon had a huge role in Armageddon and haven't shown up or been mentioned much in game since. It'll be interesting to see how they would fit into this new universr. Sareena is a minor character that has potential. Beyond Armageddon, she had a cameo role in the story mode of MKX. D'vorrah and Kotal both returned in MK11, so this would make a third game if either of them returned. Ferra?
Geras, I could see returning due to him being a fixed point in time. Dope overall character and he could really challenge or help Liu Kang given he has knowledge of previous timelines.