What could possibly be a fifth type I'd want to add to all of this you might be wondering?
Take a look at this seemingly random group of Pokémon:
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I saw some potential in "weird" Pokémon designs. At first I wasn't sure what exactly I'm looking for, but the contrast of Steel and Ghost was on my mind.
Both feature some biologically radical creatures (a lifeform made of gears or magnets? lifeforms that don't have mass and go through walls?), and then it occured to me there could be another type inbetween, playing a role opposite of Steel.
You've probably heard of the "amorphous" egg-group. Basically it includes Pokémon that don't have a fixed body structure. That's the first aspect that fit nicely as a real tangible contrast to Steel (unlike the ethereal nature of ghost-types), creatures who have deformability...plasticity in their nature to some degree.
A generic example Pokémon design of the type I was looking for, could be described like "an orange rubber blob with tentacles" xD
But in addition to that (I mean, not all Steel types are abstract machinery-inspired designs either, some are merely covered in armors made of metal alloys for example), the type would also include aspects of wearing and using unnatural materials (with, but not exclusively, a focus being polymers) or toy/appliance-like appearances.
I present to you my most WTF made up Pokémon type so far:
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The main gripe I have with it, is that I can't include Mimiyku and Rotom in the selection due to them being distinct dual types already.
I wanna point out some of the japanese names here.
Night Slash is originally called "crossroad killing", and Dark Pulse is literally "Evil Pulse" (you can also notice that the description talks about an aura of horrible thoughts).
So there isn't really much literal "darkness" in the current Dark type to begin with. Which might actually be a good reason to add one (it would make sense to address these glaring intentional mistranslations first though..)
But in that case, the existing competition you are looking at is the Ghost type, as they are the prominent night-time creatures and that type is where all the relevant moves are (half of the ghost type moves simply have "shadow" as the first part of the name)
Now I wonder how "Dark" can be fixed though... renaming it to Trickster? Sounds a bit long. How is Shady type? Dubious type?...Coward type? Mean type, Vile type...
Ah I see where you're going. Bewear, Darmanitan, Magmortar and Electivire?
Emboar, Rhyperior, Golem, if they weren't all dual types. Am I on the right track?