The approach I would do is the following.
Stripe all classes/skills down and set a baseline for damage and healing for abilities.
Using a modifier the damage and healing is adjusted if the skill is a DOT/HOT, AOE or Single Target, a targeted skill or a ground skill, buffs, debuffs, CC effects, etc...
All damaging abilities basic morph does more damage but requires more resource to cast and does not offer CC effects or buffs. Morphs provide CC effects, buffs or debuffs at a cost of damage reduction.
All classes within class skill have access to three major buffs: Major Resolve, Major Sorcery / Brutality, and Major Prophecy / Savagery.
All classes have a class skill that is a finisher with a 300% damage increase.
Each class would have a group healing/shielding ultimate that provides a major buff to their group: Major Protection, Major Berserk, Major Courage, etc...
All classes would provide a minor group buff from using specific abilities:
Cross Class ultimate major buffs: Two classes use specific damaging ultimate and hit the same target both players get a major buff
Cross Class skill minor buffs: When two classes uses specific damage skills and hit the same target they both get a minor buff.
Casting/Channeling Time: Skills with instant casting time do significantly less damage. Skills with a casting/channeling time can easily be interrupted but are top damage dealing abilities.
Weapons skill morphs contain debuffs, no class skill can do debuffs. Each weapon will have access to one major debuff and one minor debuff.
Vampire and Werewolves: Make them the same expect one is magic and one is stamina driven. Both would have Major Resolve, Major Sorcery / Brutality, and Major Prophecy / Savagery when transformed and each would be able to provide a major buff and minor buff to the group along with being able to get the additional class cross minor buff skill. There is one morph from a skill for each that offers a major debuff. WW and Vampires cannot be in the Fighter Guild.
Guilds: Static stat boost and additional skills.
This idea is my initial thought on how to fix the game. Maybe each class should have one major debuff from an ultimate and minor debuff from a class skill that isn't found within the weapon skill line and is not shared among the classes.
Best way to ensure balance is limiting buffs and debuffs and ensuring that all classes have a role in groups and can be effective in PVP.
Right now DK, Wardens, NB are over tuned due to their self buffs. As for Debuffs its Wardens, DK, and Necro lead the way. Right now Templars and Sorc especially are on the lower end of group utility in the buff and debuff and it greatly impacts group balance and how many players play a class.
I don't expect anything to change other than more minor adjustments here and there causing further imbalance in the game between all the classes. But at least a change like this would greatly improve balance in the game and make each class unique and add to the game class interaction making it important to bring along different classes.