Maybe something like a 4th spec that is based on another class - the 'light form' paladin could be the new paladin tree. Lightform as a second healing spec or something as there is only one class with 2 healing specs at the moment.
I could think of more that aren't desperate overreaches as well as their playstyle and themes are close enough to justify some ability crossover
Paladin/Priest - Focus on 'alignment' magic
>Lightform for paladins and Balance form for priests shadow and light together as there isn't a spec that balances them currently
Shaman/Druid - Devotion to nature
>Wild Endurance (shaman tank spec based on the big health/armor druid style) and Elemental form for druids (not sure of playstyle)
Rogue/Demon Hunter - Mobility and dual wielding
>Chaos focused spec for rogue that incorporates chaos damage, Demon Hunter stealth spec that incorporates stealth and 'surprise' elements
Death Knight/Warlock - Shadow Magic and summoning
>Shadow spec for death knights that utilizes shadow spells in addition to melee attacks, necromancer for warlock that replaces demons with variety of undead
Warrior/Monk - Focus on martial proficiency, power through training and discipline
>Kensai type warrior that enchants their weapons with ki power like a DK uses runes and a weapon master monk that incorporates weapon strikes into their combat in more than the current aesthetic way
Mage/Hunter - Ranged attacks with a arcane influence (weakest combination for sure as Hunters aren't focused on arcane, but not horribly far off... arcane archers for example exist in many games)
>Alternate mage pet spec for mages and a more magical focus on ranged attacks for hunters
Obviously some of these are a bit weak because I'm trying to make some of them fit without spending all day thinking about it, but I think there's enough good here to justify considering the idea.
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I like that actually - that's a cool idea. Additionally, world quests could be optionally be endgame content instead of a grind. Make the quests hard and have them reward upgraded loot to farm between mythics/raids.
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That sounds like an excellent idea, sounds difficult to balance, but I love the concept.