Hello everyone!

What if fighters had an ability that gave him/her a chance to ignore the AC bonus of plate armour? Also, do slashing weapons get a penalty to plate? Do bludgeoning weapons get a bonus?

In Throne of Bhaal, probably nothing, as AC becomes irrelevant at that point in the game.

Otherwise, it would make the Mage's defensive advantage over a Fighter even more extreme.

(Seriously, except early in BG1, Mages are better tanks than Fighters already; this change would just make the discrepancy bigger.)

I've seen the "mages over fighters as tanks" a few other places and I'm a little skeptical. In BG 1+2 pretty much every caster sneezes dispel magic. Which means you'll have to re-cast all of your defensive spells.

Icewind Dale 1 is a different story. In my HoF playthrough, the only dispel magic that was thrown at me was at the main campaign's final boss fight.

Edit: I just remembered something. Mages can't use swords. So that wouldn't increase an advantage over fighters.

In BG2, if Dispel Magic is a problem, just cast Spell Immunity: Abjuration. Now that spell will no longer affect you.

In IWD1, I distinctly remember being dispelled at the end of the third dungeon (the short dungeon after the Vale of Shadows). Specifically, my shapechanged Fighter/Druid had her natural weapons dispelled, but she was still shapechanged, leading to a wierd situation.

Also, swords don't have any relevance from a defensive standpoint; they're only useful if you are actually attacking.

The dispel thing is probably because the IE engine can be random sometimes. At that dungeon I found a pair of boots that give immunity to entangle. When I got to the end of the dungeon the clerics there kept casting entagle; so they would be rooted and my fighter/thief was free prance around like some...I can't think of a way to finish that sentence.

I just assumed that happened to everyone. It's like those clerics get a pay raise or something if they cast entangle several times per day.

Why? Weapons don't get bonuses or penalties. It's the armour that affects protection (note that bonuses are separate from the base AC protection):

Leather armour grants no specific bonuses, and has a penalty to piercing and missiles.
Studded leather grants small bonus to slashing, piercing and missile.
Chain armour grants bonus to slashing and penalty to crushing.
Splint armour grants bonus to crushing, piercing and missile.
Plate armour grants bonus to slashing.
Full plate armour grants bonus to slashing, piercing and missile. Unless they have such protection added, like Drizzt's 'Defender'.

If I understand this correctly, a character wearing leather armor and under the effects of an armor spell such as Armor or Barkskin is worse defensively than the same character without the leather armor?

Also, does the bonus or penalty to missile defense from armor stack with the one from the Shield spell?

There's no need to apologise. My question was just that, a question -- I didn't see, and still don't see, any reason for such an ability, but that doesn't invalidate the idea. Yes. There is absolutely no reason to wear leather armour, especially in BG1 with the prevalence of missile and spear enemies. Armour (spell) is cumulative with DEX and Shield bonuses, though I'm not certain about physical armour stacking.