@Vulgar I like a lot of series and things that have not devolved into multi-verse, time travel, or other unnecessary plot contrivances. They are not "natural escalations."
A big reason for these plot devices is that a good story has a beginning, middle, and very importantly, an END. People don't want things to end anymore, so story tellers run out of ideas for their world, or they've blown they're load and feel pressured to up the ante or bring back certain things from more popular times in their respective media.
It's trite. I've liked certain movies and games that involve these kinds of stories, but usually they're just lazy.
And on the dark tone?
This is my childhood MK roster select:
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This is MK1 at launch:
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It shouldn't take words, but MK3 had flavor. It attempted to be scary or distinctly dark. There are THREE characters SMILING on Mk1's.
It's honestly a lot to do with Music, also. The music used to be tense and eerie and now it's triumphant and epic.
Mortal Kombat is and always will be dark, it's in its nature. But it used to hit that theme better. Even the story beats are far more optimistic, and characters like Cassie Cage sort of feel like hero protagonists when Mk used to kill those or turn them evil or have them actually lose.
There's levels to it that I think maybe you'd have to be media literate to appreciate.