I think for boss you could use Argus, who is Rain's father and Edenian God.

I also thought about Onaga as playable character since he was the first Outworld Emperor.

I wouldn't mind an ancestor of Johnny. An ancient Greek character would be interesting on his own anyway, since Greek mythology is, like, the most well-known and popular.

Kahn killed Onaga like a hundred-thousand years ago. A game about Great Kung Lao and Goro would have to be set only 500 years ago.
And Argus doesn't really have any reason to fight anyone or be involved...unless he were fighting Daegon, and rather than saying "Daegon killed his father...or so he thinks" in his bio, you could make that his ending instead. (Which perhaps might also suggest the need for Shinnok or a BotS member to appear in the game as well since Shinnok is the reason Daegon "killed" Argus and Delia, he became a mentor figure to him somehow after he woke up has been manipulating him Palpatine/Anakin-style ever since, including telling him not to trust his parents, that they'd take Taven's side during "the quest".)

I totally agree with the idea of a prequel. We always hear about Shao Kahn ruling for years, previous MK tournaments, The Great Kung Lao, ect. I'd like to see a game that focuses on the Rise of Shao Kahn. So much potential for a prequel that it's ridiculous. Unfortunately a lot of fan favorites like Sonya, Jax, Kano and others wouldn't be in a prequel.

I love the idea. I would love also see Siro and Taja in there as well.

This sounds like a great idea, but I might like it even more if it somehow spanned multiple generations in the story mode.

For example:

First third: Shao Kahn's konquering of outworld and it's various realms, Edenia, Zaterra, etc.

Second third: 500 years before the current game, telling the story of Great Kung Lao that you outlined.

Final third: the events directly leading up to the current game, such as Sub-Zero's murder of Scorpion, etc.

That's just my two cents.

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