Looking into the future of 86+ I think the next class will have to include a tank spec due to the need for tanks. It also should have some core elements both to draw people to the game who hadn't considered it before and to satisfy current players who pick it up with its flavor and playstyle. I have to admit I have trouble picking just one option so I will just sketch up two of them.
Option #1, the Rune Warrior (RW) Standard Class.
This is based off the Runemaster character from Warcraft lore satisfying the lore geeks who will be thrilled to have it. The playstyle with its focus on up close and personal brawling should please those interested in it. Including an unarmed(ish) hand to hand specialist will satisfy the many people who have longed for a monk style character. The armor design will need some smoothing out but will be an interesting new twist on loot without needing to itemize new gear. Their buffing ability could be used to fill in a minor way for 'missing' buffs (Such as a +2% stat buff instead of 5%)
- Intellect increases strength on a 1 for 1 basis.
- Spirit grants Armor Class on a 2 for 1 basis.
- Relics unless otherwise noted for range slot.
- Armor Type: Cloth
- Weapons: Dagger, Fist, 1-H Maces, Staff
Spec 1: Epic Runemaster
DPS: Specializes in crushing their foes with personal melee attacks. Talents increase mobility and their ability to deliver crushing personal blows.
- Charge Attack
- While weilding no weapons at all the Runemaster has a scaling unarmed attack that increases potency with level.
- Frenzy Ability
Spec 2: Lone Wolf Runemaster
Tank: Their focus is personal survival and enhancing their own ability to tank damage and remain standing.
- Can equip and use bows(guns)
- Increased Armor Class when weilding a Staff
- Taunt and Last Stand ability.
Spec 3; Spiritualist Runemaster
DPS: Meditative seekers of power their talent tree increases their group buffs to equal any other classes' and lets them aid their party.
- Spirit Strike - Melee range instant heal (usable on self)
- Magic Blow - Attacks do additional damage of a random magic type.
- World Walk - Blink like personal teleport.
Option #2, the Demon Hunter (DH) Hero Class.
Much like the Runemaster the Demon Hunter class will satisfy the Lore lovers and interest those new to the game in working their way up to try it if presented right. Having it open to multiple races might be a sticking point but it is explainable that with the fall of the black temple all Illidan's students scattered and starter trading their teaching for whatever they could get. Much like how certain martial arts were spread.
Special: They have a shirt item slot that removes the chest graphic of their armor so their tatoos can be shown - with appropriate halter top for the ladies. Also coveres the face graphic with a blindfold. I'd like it if there were more items added for the flavor but this would be enough to make them distinct.
Tank: by taking the form of a demon and using its tough body the Demon Hunter becomes nearly immune to attacks allowing him to shrug off the most lethal of attacks and occasionally start forest fires.
- Damage reduction and additional stamina in demon form.
- AoE Fire attack
- Can leap into the air and pull all the enemies near where he lands to him annoyig them greatly.
Spec 2: Specter
- Gain attacks that can be used with a ranged / thrown weapon.
- Gain Stealth (More surprise/mobility than ambush)
- Create a shadow clone that mirrors the character and uses all the abilities on the hot bar.
Spec 3: Channeler
Ranged: By calling upon the powers of the demons the Demon Hunter can empower his strikes with various energies to consume his foe.
- Shadow Wings that enhance spell damage in melee range and reduce spell damage taken.
- AoE Shadowbolt and drain life.
- Parasitic shadow spawn. They infest someone, explode out doing damage and look for new hosts.
I have more ideas like the Mage-Priest but those two would add a lot of flavor and utility to the class balance without really throwing it out of whack too badly. I personally most favor the Demon Hunter - both because I think its an awesome concept and the fact Blizzard promised them to us in the game book for the original World of Warcraft game.