Currently, buffs / debuffs have a Minor and Major version. Each creature can have 1 instance of both Minor and Major, which stack. For example, Minor Berserk increases damage done by 5%, and Major Berserk increases damage done by 10%, for a combined total of 15% increased damage. I believe this system greatly reduces the options that players have in terms of skill sets, gearsets and playstyles. It also creates a ton of redundancies within a party, which makes pugging even more miserable than it should be.

I propose that we modify this system by removing Minor and Major effects, and instead implement a stacking feature for each status effect. For each stack, the effect of the status effect increases. For our Berserk example, instead of a fixed 5% or 10%, each stack of Berserk increases your damage done by 0.25%, for a total increase of 25% increased damage at 100 stacks.

Give each effect a standardized time such as 15 seconds, or 30 seconds, or whatever is the most correct length of time. Do not increase the duration each time stacks are added. Once a buff / debuff timer reaches 0 seconds, it ends and the new one starts at whatever number of stacks the skill generates.

Abilities that currently grant a minor version of a buff instead give 5-10 stacks, and abilities that grant major version now grant 10-25 stacks.

Effects that modify the duration or effect of these buffs/debuffs would become an effect on the creature itself. These effects will have two different values. One for being in a standard team or solo, and one for being in a large group. These effects will stack up to 10 times using the value in parentheses.

For example, the Blooddrinker set increases the damage-over-time of your Bleeds by 33%. With this system, it'd look more like this;

(2 items) Adds 833 Weapon Critical
(3 items) Adds 129 Weapon Damage
(4 items) Adds 833 Weapon Critical
(5) When you inflict the Bleed status on a creature, you also inflict the following effect "Blooddrinker: Lasts 30 seconds. This creature suffers +15%(6%) more damage from the Bleeding status effect."

The number in parenthesis is only used when in a large group, and only if multiple sources of that specific buff / debuff were applied. If only a single person in a 12 man group is wearing Blooddrinker, then the 15% increase would apply. If there's 2 players wearing this set, then the total buff value would be 2 Stacks of Blooddrinker @ 6% per stack, so a 12% increase to Bleeding damage.

Regular abilities that apply their own specific effects wouldn't be affected by this new system. (Unless you want to.). Due to the intelligent nature of Stam / Magicka costs, Players can absolutely spam abilities to boost their stack numbers.

Below is a general idea of each Buff / Debuff.

BUFFS
Aegis: Reduce the damage you take from dungeon and Trial monsters by 0.25% per stack.
Berserk: Increases all damage done by 0.25% per stack.
Brutality: Increases weapon damage by 0.30% per stack.
Courage: Increase Weapon and Spell damage by 5 per stack.
Empower: Increase the damage of Light, Medium, and Heavy attacks by 0.60% per stack.
Endurance: Increase Stamina recovery by 0.40% per stack.
Evasion: Reduce damage suffered from Area attacks by 0.25% per stack.
Expedition: Increase movement speed by 0.30% per stack.
Force: Increase all critical damage done by 0.20% per stack.
Fortitude: Increase Health recovery by 0.20% per stack.
Gallop: Increase Mounted speed by 0.30% per stack.
Heroism: For every 30 stacks, you increase your Ultimate generation by 1.
Intellect: Increase Magicka recovery by 0.40% per stack.
Mending: Increase Healing done by 0.15% per stack.
Prophecy: Increase Spell Critical rating by 22 per stack.
Protection: Reduce damage done by 0.10% per stack.
Resolve: Increase Physical and Spell Resistance by 60 per stack.
Savagery: Increase Weapon Critical rating by 22 per stack.
Slayer: Increase the damage you deal to Dungeon and Trial monsters by 0.25% per stack.
Sorcery: Increase Spell Damage by 0.20% per stack.
Toughness: Increase maximum health by 0.10% per stack.
Vitality: Increase healing received by 0.15% per stack.

DEBUFFS
Bleeding: For each stack, suffer 15 Bleed damage every one second and increase the cost of your Stamina abilities by 10.
Burning: For each stack, suffer 30 Flame damage per second.
Breach: Decrease Physical and Spell resistance by 60 per stack.
Brittle: Increase critical damage taken by 0.10% per stack.
Chilled: Reduce Movement Speed by 0.30% per stack. If this effect reaches 100 Stacks, remove this condition, you become Frozen for 10 seconds instead.
Concussed: Reduce the damage of Light, Medium and Heavy attacks by 0.40% per stack.
Cowardice: Reduce Weapon and Spell damage by 5 per stack.
Defile: Reduce healing received by 0.15% per stack.
Diseased: For each stack, suffer 15 Disease damage every one second and increase the cost of your Magicka abilities by 10.
Enervation: Reduce critical damage dealt by 0.15% per stack.
Lifesteal: Each Attack you make heals your target for 6 health per Stack.
Magickasteal: Each Attack you make restores 6 Magicka to the target per stack.
Maim: Reduce damage dealt by 0.10%.
Mangle: Reduce maximum health by 0.10% per stack.
Poisoned: For each Stack, suffer 15 Poison damage and increase the damage you suffer from Poison abilities by 0.50%.
Sundered: Increase damage taken by 0.10% per stack.

Fixed
Frozen: You cannot move, break free or Roll Dodge. You suffer 20% more damage from Heavy Attacks. If you are frozen for more than 3 seconds, you gain 50 stacks of Brittle. If you are hit by a Heavy Attack while at 40% health or lower, you shatter, suffering x,000 damage and ending this effect.

The reason I propose this change is 3-fold;

1.) I believe as the game goes on, more and more sets will be shut off from players as more effective and efficient sets become available. By removing the dichotomous Minor / Major restriction, Players will have a MASSIVE gain to their end-game options.
2.) Right now there are a TON of unseen redundancies in groups because of the extremely limiting Minor / Major system.
3.) It adds another layer of cooperation to group play.

Additionally, if you wanted to, you could class Weapon Types (Like you're doing to armor) and assign standardized debuffs to each type. Something like

Bladed: (Sword, Greatsword): Bleeding. Light Attacks inflict 1 stack, Heavy inflicts 3. Double for 2h or DW.
Edged: (Axe, Battle Axe): Sundered. Light Attacks inflict 1 stack, Heavy inflicts 3. Double for 2h or DW.
Blunt: (Mace, Maul): Concussed. Light Attacks inflict 1 stack, Heavy inflicts 3. Double for 2h or DW.
Piercing: (Bow, Dagger): Breach. Light Attacks inflict 1 stack, Heavy inflicts 3. Double for Bow or DW.
Inferno Staff: Burning. Light Attacks inflict 2 stacks, Heavy inflicts 6.
Ice Staff: Chilled. Light Attacks inflict 2 stacks, Heavy inflicts 6.
Lightning Staff Maim. Light Attacks inflict 2 stacks, Heavy inflicts 6.