Bug- Defensively resists common fighting and ground type attacks, but weak to common attacking types like fire and rock. Offensively they have decent coverage with a few common resists. Since the most commonly used Bug-type attack is U-Turn, they don't have to directly worry about moves resisting it as they instead force a "do or die" situation for Psychic/Dark/Grass types, as we would know with how Scizor generally works. I say we can leave it alone, though maybe one less thing resisting it might be a good idea.
Suggestions: None
Reasons: Good enough: defensive ones exist+offensive ones. Doesn't need help
Dark- Defensively resists Dark and Ghost while being immune to Psychic: pairs up with the ghost type to have no weaknesses. Offensively has pretty neutral coverage against the majority of types, while being resisted by commonly used Fighting/ Steel types. Usually used in the form of Crunch/ Pursuit/ Sucker Punch, most commonly used in Pursuit to help trap what would otherwise be incredibly troublesome ghost and psychic types.
Suggestions: Dark is Neutral vs Steel.
Reasons: Gives actually a good reason to use Special Dark moves, to help prey on the lower sp. defensive stat of most steels. Never did make real sense, anyway.
Dragon- Defensively has a plethora of resistances, some of which include water, fire, and electric. Also has a resistance to grass that really hinders it's ability to be a good attacking type. Offensively, Dragon is superior to every other type: hitting itself for super-effective(giving it more reason to use it), while hitting everything for neutral EXCEPT for steel, hence the centralization between Dragon and steel types in the current metagame.
Defensive Suggestion: Dragon no longer resists Grass and Electric.
Reasons: Gives every Dragon type more weaknesses, meaning they hinder you more defensively.
Offensive Suggestions: Dragon is resisted by Normal.
Reasons: Other than Blissey and Snorlax, what Normal types reside in the current OU metagame? This gives pokemon like Staraptor and Tauros a chance to be a good physical wall, as well as decreasing the centralization towards steel types(we have more choices to combat dragons! yay!).
Electric- Defensively resists common Steel and Electric attacks, but weak to the super-common Earthquake. Offensively hits a lot of types neutral, but common ground types are immune to their onslaughts. Hits "bulky" waters super-effective.
Defensive Suggestion: Electric now resists Ice.
Reasons: Ice is a really good attacking type: it hits a lot of common types super-effective and nothing is ever immune to it. Electric needs more of a push as a defensive-type, and it being able to resist the ever bolt-beam combo might be enough to do it. Scientifically, trying to freeze a magnet doesn't work: in fact, it turns it into a super-conductor, making it only more powerful. So it makes logical sense as well.
Offensive Suggestion: Electric hits neutral against Dragon.
Reasons: Already explained.
Fighting- Defensively resists common Rock, Dark, and Bug attacks. Weak to non-common Plying and Psychic attacks. Not terribly broken, but hard to capitalize on: thankfully, the amount of purely-defensive Fighting types are limited. Offensively hits Steel, Rock, Ice, and Normal super-effective, keeping Steel and Normal in check but very much hindering the use of Ice-types as defensive pokemon. Resisted by Flying, bug, and poison types, and is immune to by ghosts. Very good, but not terribly abusable. It's just enough to keep types in check.
Suggestions: None
Reasons: Really a good type as-is. Doesn't need any fixing.
Fire- A mixed bag defensively. While resisting four very common types such as Fire itself, Steel, Bug, and Ice, it has two insanely common weaknesses: EQ and SR. Offensively it can stand with the best of them, hitting Steel, Ice, Grass, and Bug super-effective but resisted by Rock, Water, and Dragon.
Suggestions: None
Reasons: As far as fire types go, they are offensive. And the few defensive ones are part flying, and flying gets a boost in lack of rock weakness so SR isn't the biggest pain anymore. Torkoal will be fixed...
Busy. more to come