Personally I wouldn't want the dead characters returning other than Liu Kang and Motaro (for story purposes) and a few iconic characters like Sub Zero and Kitana.
Forces of Light
Raiden
Fujin
Kai
Shujinko
Johnny Cage
Sonya
Bo Rai Cho
Kenshi
Ashrah
Forces of Outworld
Mileena
Ermac (King Jerrod)
Li Mei
Goro (appearing only in story mode)
Kintaro (appearing only in story mode)
Sheeva
Forces of Netherealm
Shinnok
Quan Chi
Scorpion
Noob Saibot
Motaro
Liu Kang (resurrected)
Baraka
Reiko
Kano
Jarek (appearing only in story mode)
Mavado
Hsu Hao (appearing only in story mode)
Reptile
Sektor
Cyrax
Rain
Tanya
Kia (appearing only in story mode)
Jataaka (appearing only in story mode)
Sareena (appearing only in story mode)
DLC and others(characters not related to story)
Sub-Zero (human)
Cyber Sub-Zero
Smoke
Kitana
Jax
Sonya
Havik
Kung Lao
Goro and Kintaro could be playable via special modes only and not online. I wouldn't mind Jarek and Hsu Hao being playable provided NRS gets their creative juices flowing and molds those characters well.
I would like motaro to be one of the sub bosses in the future along with a hellbeast lord of some sort. And Shinnok should be the main boss with 3 forms (somewhat like Ogre in Tekken) as how Bruce suggested in the Shinnok thread.
Boss Progression (from lowest to highest)
Quan Chi - perhaps with double the health bar.
Possesed evil Raiden - an evil version of Raiden whose soul has been possessed by Shinnok, his electricity, eyes, blue portions of his attire turn blood red and he is OP, dealing insane damage and taking less compared to normal characters.
Spectre Liu Kang - A spectre version of Liu Kang who quan chi resurrects in similar fashion to how he resurrected scorpion. His body is pale containing the burnt marks when he was killed by raiden, white eyes, flowing hair, and flames engulfing his hands occasionaly. He too could be OP same as Evil Raiden.
Motaro - Like the same cheap ass he was in Mk3.
Hellbeast - The hellbeast boss from MKSM. He could be a towering bohemoth, standing as tall as perhaps Onaga, his attackas can cause flame damage that slowly eats away your health overtime.
Shinnok - He could have 3 forms. His normal form, his demon form and a more gruesome form which we have not seen, each of them having their own full health bar. (Props to Bruce for the idea.
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