This approach creates great user-friendliness and less time-investment vs active-skill or charge/cast system: the game is simplified to reduce switching tactics/skills as most skills are always-on bonuses to main attack.
Instead of requiring switching between dozens of auras,buffs, states,active charges(of skill/item) and skills themself,
The following system is proposed:
1.Minimal number of active use skills(about 3-5 max)
A.Single-Target Missle.("The Attack Skill")
B.AreaOfEffect Skill Missle(or add area of effect damage from passive to A)
C.Melee attack
D.Utility Skill
2.All other skills are converted into passive skills:
A.Auras= Converted to permanent aura which stacks with all other auras.
B.Classic Passives = Same or more "always-on" mode.
C.Attack Boost skills = Boost and stack to skills in 1.A,B,C
D.Defense Buffs/Skills = Provide static bonus/buffs, "always-on"(autocast) aura.
E.Spells = Become autocast to nearest monsters or apply their bonus to skills in 1.A,B,C
G.Debuffs/Curses = Autocast to all monsters/enemies in range.
H.Other skills = Either autocast or apply their features to all attacks.
Synergies/Bonuses work normally, except "always-on" with all skills passive.
Attacks become autocast to nearest enemy or pointed to direction.
Mana costs of all skills become zero(mana system could be used as shield before HP, with free "permanent mana shield" for all classes).