Rogue 4th spec Worgen Slayer for Horde and Forsaken Slayer for Alliance.
It duel wields cross bows, its attacks are silver bullet pistol, aspergillum morning star, woode n steak strike, Wurfkreuz, slaying shot, slayers volley, slain barrage, and has a raven minion.
DH - You mentioned the third spec. 4th spec similar to DH in Diablo, dual one handed cross bows, cal trops, bombs, able to have a demon dog helper on a cooldown.
Hunter - Change BM to a tank spec, with controlling pet tank being the priority, have eyes of the beast as a tank cooldown. Add a 4th spec that is similar to current BM dps, but call it Zoologist.
Gonna go through OP’s list and give my feedback:
Sword+Board DPS Warrior It’d be really hard to make this spec different enough from Protection Warriors. Gladiator Stance makes more sense, but Blizzard doesn’t like blurring the lines between specs too much.
Void-Themed Paladin Adjusting Paladins away from the Light theme goes against established lore and class fantasy pretty hard. If Paladins were named “Heralds” instead, this could work, but they’re not.
Mounted Marksman Hunter Having a “Divine Steed”-esk cooldown for the Marksman Hunter makes more sense than having an entire spec revolve around this concept.
Earth-themed Shaman Tank This is a fan-favorite 4th Shaman spec that’s been floating around for a long time, though Blizzard has yet to act on it.
Blood or Illusionist Mage Blood Mage has some thematic precedent in the lore via the San’layn, but their abilities might bleed (pun not intended) too much into the Warlock’s existing toolkit. Illusionist Mage might be really challenging to develop a combat/damage dealing aspect without it bleeding into Shadow Priest and/or Arcane Mage.
Warlock Healer Another 4th spec fantasy that’s been suggested here and there, though not as popular. It’d be interesting to see what could be done beyond Drain Life, Health Funnel, and Health Stones.
Death Knight Necromancer Plate-wearing Death Knights as a caster doesn’t make a ton of lore-sense, and this fantasy is already fairly realized through the Unholy spec.
Demon Hunter 2-hander DPS There’s some precedent in lore for this to be a thing, but it’d be hard not to make it just feel like a Fel-themed version of an Arms Warrior.
Marksmanship Monk Like throwing stars and ninja stuff? Rogues already cover that with many of their abilities. A more popular 4th spec fantasy for Monk is a ranged DPS caster themed around the Jade Serpent (healers remain themed around just the Red Crane).
Cleric Priest Nearly impossible for this not to feel like the current Holy Paladin. Too much overlap in the class fantasies in the way they’d be portrayed in WoW.
Ranged Rogue It’d likely bleed too much into the Hunter class fantasy, in spite of how much fun it could possibly be.
Druid Already has 4 specs.
Some other popular 4th spec ideas you didn’t mention:
Ranged DPS Warrior - throwing weapons, etc.
Hunter Tank - Either using the pet as the tank and/or emphasizes animal aspects (and uses them more like stances).
Mage Healer - Uses time magic, but this is now covered by Preservation Evokers.
Warlock Tank - Similar to Hunter Tank, though it’d use demonic transformations rather than animal aspects.
Paladin - Zealot - a paladin spec that dual weilds and focuses on burning their foes with holy zeal, blinded in their beleifs. A talent could be named after a famous scarlet crusade qoute “show me your secrets!”
I want them to bring back the playstyle that was Survival, prior to Legion, as a 4th spec to the hunter class. A modern version that is based on the old signature abilities/effects it used to have. I played it continuously for almost a decade. There’s nothing within the class that plays like it, not even close.
What could it look like? See the link below. It has an option to be viewed via a talent calculator, in case you want to tinker with it yourself.
Druid - Dragon form, asume the aspect of a powerful melee aoe dragon. Wreck havoc up close with massive tail swipes, swipe with your claws and rend their flesh, and call on your brood to assist you in battle.
For Paladins, I would rather see a sword and board DPS spec in same flavor of Gladiator Warrior, let me feel like a Holy Knight with a sword and shield without being a tank.
Warrior - either a gladiator sword and board type or an archer dps spec. No nature theme just a archer or rifleman concept.
Paladin - Void Knight. The velves and Forsaken spearhead a new order for their factions. A branch of the Rift Blades for velves and “holy” knights for the church of forgotten shadow for Forsaken. These orders accept other races into their groups. (Mag’har and Orcs and worgen also gain paladin with these orders in mind.)
Hunter - Brute: tank spec that uses two-hander or dual wield. Pet and hunter share health and empower each other. Dead pet equals dead brute hunter.
Shaman - Earth warder. Uses earth element primarily to protect and attack.
Mage - Spellbreaker tank. Uses magic to strip away defenses and interrupt casting. Uniquely able to interrupt normally uninterruptable abilities though with a medium cool down.
Warlock - Soulbinder: takes soul energy from enemies and uses it to heal and strengthen allies. Uses the fel hunter as its main pet.
DK - death spec. Dps ranged using death runes. Since I can’t have necromancer. Summons undead constantly and uses shadow frost spells otherwise. All ranged.
DH - 2h scythe melee range hybrid with more than 3 buttons to press (this is good agree with OP.)
Monk - dps ranged based on crane celestial. Blasts the enemy with chi and such from afar.
Priest - Void Deacon: shadow based healing spec or Avenger: dps holy spec.
Rogue - Skirmisher: ranged dps spec using crossbow, gun, bow or thrown. Bring back thrown weapons.