Here’s the specs I think we are missing, and who I think would be the best people to fill those spots. I’m going to plug the ones I’ve made in hyperlinks, since I think the OP might be interested.
DK: Darion can fulfill the rune management that is memorable. Some people suggest Saurfang for blood.
DH: Kayn (Vengeance) was Illidan’s most loyal follower and co-leader of the Illidari. He would also be a belf rep.
Druid: Runetotem could make for a very unique, authentic solar/lunar balance druid based on the tauren mythos. Zen’tabra was the first real troll druid, and is canonically a feral druid. Broll Bearmantle completes Rehgar’s gladiator ring, and wouldn’t conflict with Malfurion artistically if he is bear-exclusive or nigh exclusive.
Hunter: Hemet could be a good petless rep for MM hunter, possibly with traps. Survival could be any number of Hunters, though Huln as a Highmountain tauren rep would be nice, since he was canonically a melee survival hunter.
Mage: Thalyssra would represent the Nightborne, and could easily represent unique arcane mechanics. Alternatively, somebody like Millhouse could bring in authentic Arcane Charge mage. Additionally, Khadgar could be implemented to represent a universalist mage.
Monk: Taran Zhu could fulfill the windwalker role missing as a close range ability-centric melee assassin. For Mistweaver, we could bring in Shaohao as a mistweaver representative, or even do an Aysa and Ji duo character as mistweaver and windwalker, respectively.
Paladin: We should clearly get the iconic Tirion Fordring for retribution paladin, being owner of the legendary Ashbringer and having an interesting history over the game (even while Uther was alive). For protection, Liadrin would be a perfect fit.
Priest: Moira completes the Council of Hammers and is canonically Shadow Priest (even if she casts holy spells occasionally for some reason). Also a dark iron rep.
Rogue: Garona would be the most obvious assassination rogue rep, being a legendary assassin and poison user. Edwin Vancleef could easily fulfill outlaw rogue.
Shaman: Nobundo is the obvious choice for Shaman, bringing in broken rep and being a very special draenei in lore.
Warlock: My only suggestion here is Wilfred Fizzlebang, since he has Jaraxxus behind him and was known as a master summoner. He could bring demonology warlock into the game, since Gul’dan doesn’t have any summoning.
Warrior: We’re done here.