Let's face it, how many of you guys actually consider defence when picking which armours to wear? If I asked you to pick between the Ironmight Plate set and the Skolver set, would you say "Ironmight. It gives more defence" or "Skolver coat for the damage bonus"? Skelly set or Snarby? Chaos users! How often do you consider ditching your offensive capabilities in exchange for better survivability? Do you find that you survive better when enemies are dead sooner? As a gunner, how often do you find scenarios where you actually are more useful with your guns than swords would ever be?

Of course it's true. Prevention is better than the cure. You're far better off not getting hit than letting your equipment absorb the damage. But sticking to that notion results in, well, put it this way. SK has a wide variety of armours to wear. Of that, only a fraction of that is what people consider to use and then a fraction of that fraction is what people actually use. Much of the armours are considered redundant simply because better defence is not as useful as more offense. Ancient Plate is not used, Ironmight is not used, Cobalt is not used. Chaos and Wolvers are what most people who have them normally wear.
Part of the problem is the way defence works in the game.

How I Think Defence Currently Works

Before I present my idea, I should probably point out how I believe defence works. I might be wrong, and please please correct me if I am, but lots of experiments can't be *that* wrong now can it?

There are four damage/defence types: Normal, Pierce, Elemental and Shadow. Normal defence ONLY defends against normal, pierce against pierce, etc. If you only have pure normal/pierce/shadow, for example, and get hit by pure elemental damage, you take the full blow.

How I Think Defence Should Work

From my experience, I believe it was OOO's intention to make Normal defence a kind of "use anywhere but don't expect it to be as good" kind of armour.
That guy in the HoH who sells Cobalt gear even mentions that whilst Cobalt gear is not great against anything in particular, it can be used anywhere. Heck, you can use the Leviathan, a pure normal damage weapon, anywhere so why not defence? As for Pierce, Elemental and Shadow (henceforth "special") were to be more specific for a lack of a better word. Something that could bypass armours better and be better at defending in some cases.

With that in mind, here's my suggestion:

Make Normal Defence useable everywhere. Make it so that it defends against ALL damage types. (Most important one)
Normal Damage can only be blocked by Normal Defence.

Special Defence defends much better against Special Damage than Normal would to a point where you can be near invincible. (Well you're not gonna be THAT invincible but you certainly kinda need to exert some effort to die)
Special Damage is not defended as easily by Normal Defence.

Make it so armours take extra damage from certain damage types (status defence bars can go in the negative, so why not damage bars?)
Enemies' melee attacks should deal either pure Normal or part Special (i.e. it always does normal damage). With emphasis on big tough monsters who are able to deal massive Normal damage. (If they don't already)
Enemies' range attacks should deal mostly, if not purely, Special damage. (If they don't already)

With this kind of system, you can then tinker around with equipment stats and monster damages to be more meaningful. Here are some examples.

Possible Suggestions/Effects On Equipments, Armours and Monsters

Normal Defence works anywhere. The Cobalt set can be given pure normal defence (maybe some defence against all special damage types) and be useable anywhere! The Ancient Plate Set can finally tank like a boss even against pure Special damage types up to a certain point of course, you would not want to tank a lot of Special damage hits with it though. Additionally, if big monsters were dealing massive normal damage, wouldn't it make sense to have armour that can counter that? Ancient Plate set gets some more love again! You're big and slow and meant to take on monsters that are big and scary. I know I know, the Ancient Plate set already tanks Trojan and Giant Lichen Colony hits already....but against anything else you're kinda gonna die quickly even against the Trojans/Colonies themselves since they do have pure shadow/pierce attacks. And this can apply to all guardian oriented armours.

Since monsters dealing melee would always deal at least some degree of normal damage, if a piece of armour has more normal defence than special, it can be a cue that that armour is meant to be used by sword users. Think of how the Wolver sets have more Normal Defence than Special.

Gunners get some love too! Range attacks are almost certainly pure special, so you can give gunslinger armours pure Special defence and become practically invincible at a distance. Imagine that room filled to the brim with Gun Puppies, suddenly, they look like what they are, cute adorable puppies, not the vicious canines that they are in huge numbers. Additionally, if the armours were given no or very little Normal defence, they can take quite a blow when hit by a monster's melee attack further promoting gunslinger strategies (of not getting close) and discouraging sword users from using gunner oriented armours. Suddenly you have gunslingers being the prime candidates to have to deal with turrets and bullet hell scenarios which honestly I think there should be more of.

The Chaos Set right? I'm somewhat disappointed it has Elemental defence. I'd expect the set to be equally deadly to use everywhere. It still gives decent defense against elemental damage. But this had me thinking. What if you could have an armour set that gave massive defence....but statuses become a major problem. Conversely, what about an armour that gave mediocre defence at best but immunity against most statuses? (This part was off topic but I'll leave it here anyway)

Remember Devilites? Recall how OP they can be in large numbers? How unmercifully powerful and accurate their staplers and 3 1/2in floppy disks are? Imagine it now, a bulky, slowed down Ancient Plate user, after dancing with the Trojan, a Devilite comes close to him and smashes him with a melee attack. It dealt part normal, part shadow damage. But then gets barraged by a shower of office chairs dealing pure Shadow damage. Devilites are frisky agile things and he's having difficulty getting to them let alone landing enough hits to kill them fast enough before he starts taking some damage. His Snarby friend steps in. He has more Normal Defence than Shadow. Their melee attacks did a bit more damage to him than the Ancient Plate user but at least he's not taking as much damage against the staplers. Then a wild gunslinger wearing pure Shadow Defence gear shows up. The Devilites melee attacks are deadly so he knows to keep his distance. The Devilites are forced to use even more staplers but it barely scratches him. His guns are able to reach the nuisance wherever they fled to and promptly finishes them off.

Would It Work?
This is all fine and dandy and makes for some interesting gameplay dynamics. But other things need to change/be added if all the armours are truly to become, at the minimum, less redundant. Assuming such a system existed, the Cobalt line/Ancient Plate would eventually be shunned (as if it hasn't already). It would be nice to get early on, it would defend decently against everything but it would be the same thing with the Leviathan, you stop using it the moment you get a Flourish, an Elemental Brandish and an Acheron. You'd use the Leviathan if there was somewhere where it's damage output is better than any special damage sword, right? There probably needs to be much more enemies that deal mostly normal damage for the armours to be useful. What about pure special damage defence gunslinger armours that are amazing at a distance? As nice as that sounds, there's far too few bullet hell scenarios and a lack of enemy "gunner" monsters for that to be really viable. You'd take a ton of damage from melee monsters and claustrophobic settings make for a quick coffin.

Final thing, there wouldn't be any incentive to get defensive armours if players don't see their effects firsthand. If you get hit, bars fly off of your body. The only indicator of exactly how much damage you took is by looking at your health bar which is not often informative. You like seeing red damage numbers, it means you're dealing effective damage. You hate seeing grey numbers, it means your weapons aren't being as useful as it should. If you applied that to how damage taken is displayed, immediately, you get this urge to find out ways to get that damage colour grey much like the same way you want to make sure the damage you do is red.

I would really like to have that sense of effective invincibility in the game. Sometimes I just wanna play tank not giving a care how much damage I dish. Getting hit without so much as taking a scratch is a really nice feeling.

tl;dr I completely ignored bombers.