This idea is only for pve, each class will have an unleashed mode which lasts for 8 seconds and cool down of 40 seconds, during the effective period, they will have the respective buff (Ideas about buff is welcomed, just a rough idea so dont take the stats seriously)

Warrior's Berserker Mode:

Max HP cuts by 50%
DMG x 2
Crit x 2

Rogue's League of Assasin:

Dodge rate becomes 100%
Crit rate 100%

Mage's Forbidden spell

Locks mana bar for that 8 seconds
Banish/Burn/Poison/paralyse (Random) all mobs and bosses within a 15 m range

feel free to input your ideas, ty