This sort of thing will be difficult to test, as we would have to make pokemon, moves of that type, and if CAP tells you anything, it is a very long process when you do it right.

First let's think of things that existing types cover. Be creative, and don't be afraid to tread on other types toes. Hell there is ground and rock. And don't be afraid if it sounds stupid. Normal, fighting, ice and water? Dark, dragon ghost? When they cranked out that first r/b, they must have been on some great drugs, I want some of that.

Here I threw some together.
Plasma/Gravity/Nuclear/Chemical/Light

Now you can try to make it make sense in a chart, but i think you can find excuses for almost anything. Like would dragon really be weak to ice? Yet strong against water grass and electricity? IMAGINATION with no restraints. Just try to make it fair if you were actually going to make a pokemon that typing. If you make a typing too strong, who cares thats why ubers and stats exist, right? Factors are you're friends.

I'll give it a whirl, I'll use plasma, because that would be cool, then muk could be plasma/poison, because muk is cool.

I'll look at the type chart, and list everything I want plasma to be strong and weak to, offensively and defensively. I will mostly base it on what is prevalent to what I've seen in OU. I also when to keep the amount of weakness/resistance balanced.

Plasma's defense-
Weak to grass, poison, pyschic, ghost, dark
strong to fire, ground, rock, dragon
Plasma's attack-
Weak against Ice, grass, pyschic, rock
Strong against fighting, steel, ghost


There, and if anything seems too farfetch'd, I'll make up a good reason. It nails those steelies in the ass, while giving those poor types a thumbs up.
What, strong against ghost but weak to ghost attacks? Haven't you ever seen ghostbusters?

To go into more detail, I will explain my reasons for each decision. Even the types you choose to be neutral to are important, so we need to think it through.

I think plasma will be a little bit more of a defensive typing, not necessarily phys or spec.

DEFENDING
Weak to

Grass

- Grass doesn't have many strengths as an offensive type as the prevalent steel/dragon types both resist it. Ground/rock/water both take it, but ground and rock seldom venture out to be hit by grass when water is more common. Water is usually bulky enough to take the weaker grass moves unless your name is swampert.

Poison

- Not much to say, poison isn't a strong offensive typing AT ALL. Giving it some love.

Psychic

- While a strong spec attacking type, psy rarely relies on the other pokes being weak to it in this gen, and then dark rears its ugly head. Plasma should be able to take spec attacks, but a psychic will hurt it if not 1HKO.

Ghost/Dark

- These offensive types aren't used much, unless STABBED. Happy Birthday ghost and dark!


Strong to

Fires

- Let's not let bulky waters have all the fun, but I am worried how much this will affect fire offensively if I am to be reducing steel usage with this type.

Ground

- Sorry ground, you get a lot of shit with flying/levitate and now this baloon stuff. But dont worry, you are still on of the strongest attacks in the game! (I hope :X)

Rock

- Why not, fuck SR.

Dragon

- Love ya dragon, but steel needs to move over.


ATTACKING
Weak against

Ice

- Try to make an argument why I shouldn't.

Grass

- Rarely a defensive grass type in OU with scizor around. Celebi will be happy.

Psychic

- Weakest defensive type in the game? Alright CALM DOWN CELEBI!

Rock

- Rock's defensively aren't what they used to be. Plus plasma will be mostly physically offensive, so sandstorm shouldn't make too much of a difference.


Strong against

Fighting

- Again, I'm not sure how much this will hurt fighting, but Fighting doesn't rely that much on surviving too many hits, but I think that is because it is comfortable with coming in on SR, and only being afraid of SE flying or psychic hits.

Steel

- The big one. I want to hit steel hard here. And I don't want its secondary type to cover all it's weaknesses. I want steel to think "I can't stay in against plasma, I need to switch." Sure there are a few steels that can take it, but I'm talking about the arrogant way to popular steels running around doing whatever they want. Here's another thorn in your side steel.

Ghost

- While ghost offensively isn't that prominent, you see these dudes that are way to hard to kill all the time. Eat some plasma rotom and shandera.

This will be subject to change according to opinions and what happens in B/W. And with making a new type comes making new pokemon, which will be very difficult to theorize on, but I will relish every painstaking moment. I will start with muk later once I put this under the grindstone.

CRITIQUE!

Someone's been into physics and chem...
Plasmas are superheated ionized gases...
Fire and Electric cover it pretty well...
Also the types offensive & defensive capabilities are completely centered around "pulling" unused types from the edge. I really disagree wity that; you should focus on the new type, not the metagame.

Wild:
Some "normals" are wayyy more beast-like than normal

Defensively:
Resistant to Fighting, Ground, Ice, Rock
Weak to Electric, Psychic, Ghost, Poison
Immune to Dark

Masters of survival and scaling terrain of sorts, they have a weakness to things uncommon in the wild. Their senses are beyond most Pokémon except the Psychic type because they don't have the intelligence of them.

Offensively:
Super Effective against Dragon, Fighting, Bug, Wild
Not Very Effective against Electric, Ghost
Nullified Completely by Psychic

Really good against aggressive opponents, but have trouble fighting against things that don't encourage the use of fangs or claws to attack.

Light:
'cause it's needed

Defensively:
Resistant to None
Weak to Dark, Flying, Steel
Immune to Fire, Grass, Water, Fighting, Wild, Dragon, Bug, Poison, Ground, Rock, Psychic, Ghost, Electric, Ice

Offensively:
Super Effective to Dark
Not Very Effective to Flying, Steel
Nullified Completely by Fire, Grass, Water, Fighting, Wild, Dragon, Bug, Poison, Ground, Rock, Psychic, Ghost, Electric, Ice

Think about it.

I think if we make types they shpuld make defensive cores eg
f/w/g
steel/dragon
psychic/fighting/dark
and also light is broken.

hell yeah just like steel and dragon

While I like the idea of Wild (Okay, I LOVE the idea of Wild), why Light? If it's as a counterpart to dark, then it doesn't make sense- Dark isn't the type of darkness, persay. It's the type of dirty fighters. Sucker Punch (ambush)? Payback? Theif? Pursuit? Knock Off? Bite? All methods that would be considered cheating in a fair fight. And if you're looking for a counterpart to that, the fighting type more-or-less fills that role- that's why it trumps dark, you see. Fair fighters will always win against cheaters (well, maybe not in the real world, but let's not go into morals). That's also why Psychic doesn't affect Dark- hard to read the mind, and thus the plans, of a fighter that doesn't have any plans. If that's somewhat confusing, here's a breakdown of what the inside of the minds of a skilled fighter and a street thug likely look like in combat;

Fighter: First, I should scout out their offense. If they seem to be someone that likes to stay on the offense, then I should find a way to slow them down. Ways to do that would be... (etc)

Thug: First I'll go in, then I'll do a little of this, then a little of that, then a little of this that. Yeah, that should do it.

Light is so broken. Slap a toxic on and go to town, plus dual Light/Steel resists every type in the game, or is immune. Broken much?

WOW

All right. So here's the type I came up with:

Devil-type

Devil represents pure evil (and I mean actual evil, not just vague sinisterness like that of the Dark and Ghost types.) Make no mistake about it - Dark is simply "dirty fighting", while Devil is true darkness - darkness as in "friggin' evil." The Devil Pokemon seek to take control of the world and turn it into their own infernal playground - you could say that they're the opposite number to Arceus. They're clever little buggers who are at their strongest when they can get others to do their bidding for them. This is what their strengths and weaknesses are based on.

Devil's defense
Weak to Normal, Light, and Mystic
Strong against Dragon, Dark, Ghost, Psychic, Poison, Fire, and Ice
Devil's offense
Weak versus Normal, Light, Mystic, Dark, and Steel
Strong against Dragon, Ghost, Psychic, Poison, and Fighting

And here's my reasoning for each of the weaknesses and resistances:

Defensive Weaknesses:

Normal: Normal-types are pure and innocent. This innocence is all but literally painful to the Devil-types, because they just can't comprehend how anyone would be uninterested in the power, money, hookers, etc. that they're offering!

Light: Light is a powerful symbol of righteousness, and as such, Devil-types find it to be very painful. They literally can't stand it!

Mystic: Mystic is the embodiment of virtue and justice. It has no tolerance for the existence of Devil-types, and will smite them on the spot before they can even begin to talk their way out of trouble, let alone try to tempt the Mystic-type to the dark side!

Defensive Strengths:

Dark: Devil-types pretty much invented deception. They know every trick in the book when it comes to sneakiness, as well as several that aren't.

Ghost: Devils aren't easily scared. When a Ghost-type tries to startle a Devil-type, the Devil is merely amused at its pathetic little antics.

Psychic: Mortal minds are dragged down by the needs and wants of the flesh, and as such, they just don't have enough willpower to overcome a Devil-type's raw evil intentions.

Dragon: Devils are crafty and savvy enough to stop a rampaging dragon in its tracks. How do they do it, you might ask? One word - bribery.

Poison: Devils exist beyond the mortal plane. As supernatural beings, they're pretty much unperturbed by poison. Poison is only poison because it wreaks havoc on biological systems; devils are more than biological.

Fire: Devils are the masters of Hell. Hell has plenty of fire, so fire really doesn't bother Devils at all.

Ice: Hell is also a place with plenty of ice - in fact, the core of Hell is a realm of near-absolute-zero temperatures! While most of Hell is hot, Devils are nonetheless well-adapted to cold climates.

Devil: Devils are smart enough not to fall for their own tricks. After all, they're already the epitome of corruption - there's no way they can be corrupted further!

Offensive Weaknesses:

Normal: As mentioned above, Normal-types are innocent. As a result, attempts at temptation tend to go right over their heads. "You can keep your funny green paper things, I can't see any reason why I would want them."

Light: As mentioned above, Light embodies goodness, and so Devil-types are all but powerless to overcome it.

Mystic: Ditto. Mystic-types, being virtue, justice, and righteousness incarnate, will instantly reject any offers that a Devil-type makes, and so the Devil-types are unable to taint them.

Dark: If there's one thing that a Dark-type isn't, it's stupid. Dark-types, being skilled practitioners of deception, can easily recognize attempts at deception made by others, and will refuse to take the bait. Dark-types are more than street-smart enough to know that the Devil's offers always involve some sort of catch. There's gotta be something in it for the Devil, right? Hell, those scumbags can't even be trusted to hold up their end of the bargain.

Steel: Steel-types are secure in their own sturdiness. Power? Who needs it? They can take on just about anything! Money? They're freakin' made of precious metals, money is worthless to them! Hookers? Eww, take those disgusting fleshbags away!!! Simply put, when a Devil-type tries to tempt a Steel-type, the Steel-type says "Thanks for offering, but I just don't need whatever it is you're selling."

Devil: Like I said, Devils can't be corrupted any further than they already are. If a Devil agrees to work for another, it's because they shared goals anyways.

Offensive strengths:

Fighting: Fighters, while honorable, are still susceptible to being taken in by promises of power. However, the honor of a Fighting-type is also the reason that Devil-types don't resist the Fighting-type - Fighting-types are a bit better at realizing that they've been duped than the other types that Devil is good against, and when they realize that they've been led down the road to Hell, you'd better believe that they'll hunt down the Devil and kick its ass eight ways to Sunday! (It's also for this reason that Devil-type moves will activate the Justified ability.)

Poison: Poison is by nature a corrupting force. Devils are even more corruptive than Poison, so they essentially corrupt the corruption.

Dragon: Dragons tend to be ludicrously greedy. This makes them way too easy to bribe. As P.T. Barnum said, "There's a sucker born every minute!"

Ghost: Ghosts enjoy playing tricks. Since Devils are highly malevolent, they have an easy time bending Ghosts into becoming mere devices to enact their will. Basically, the interaction between Ghosts and Devils is like that between an ordinary clown and The Joker. The ordinary clown is gonna be shredded.

Psychic: Devils are skilled at exploiting the flaws inherent in mortal minds. In fact, exploiting psychological quirks to make even the most brilliant of minds into their hapless puppets is a Devil's favorite pasttime. (It's for this reason that Devil-type moves will activate the Rattled ability.)

The Devil-type's role in the type Chart is basically to give Normal something to be good against. I came up with it all the way back in Gen I, which is part of the reason why Devil's strong against Psychic and Dragon - those types just aren't weak against a whole lot.