For me beginning with Horde going to Alliance:
Gunther Arcanus - A Forsaken/Lich Necromancer. In Classic, we were given a questline to recruit him into the Forsaken. However in Cataclysm and to this day post Battle for Lordaeron, he just chilling on his little island and is involved in no quests whatsoever.
Fenris Wolfbrother - The last we heard of him was in the Beyond the Dark Portal novel where he gave the Scepter of Sargeras to Teron Gorefiend and he had returned to Outland. After that, he makes no further appearances in the story aside from WoD where we meet his AU counterpart.
Hatock, Groton, and Skal - These are Fenris Wolfbrotherās three sons. We meet them in WoD. However, we donāt know whether or not they even existed in the prime universe.
Thalen Songreaver - The last time we heard of him was in War Crimes where he helped Garrosh escape from jail.
Jevan Grimtotem/Stormsong - This was the tauren who tipped off Baine that Cairne had been killed and that the Grimtotem were coming to kill him. He has never made any appearance in the game nor has he been involved in the story after being introduced in The Shattering.
Volājinās Children - Volājin has 3 children according to the lore, one of whom we met in the game named Yenniku.
Magatha Grimtotem - So Iām curious as to what Blizzard has planned on doing with this character. Is she going to be given a redemption arc or something or will she at long last go the way of the loot pinata?
Mugāthol - An Ogre that has taken up residence in Alterac since having been mind controlled by Sylvanas Windrunnerās banshees. In the fourth war, it was revealed that the ruins of Alterac were taken by the Horde. As it stands, Mugātholās whereabouts are unknown.
Meādan - Yep I know, this one is controversial. Blizzard once stated that they wanted to add him into the game. However, the employee who stated that left Blizzard in 2016. Iām curious as to what his whereabouts in the story are right now or if he is even cannon at this point.
Tammis and Clannia Foxton - The last time we ever saw them was in the Lord of the Clans novel. Thrall had ordered Grom Hellscream to give their daughterās necklace to them after the siege of Durnholde. However the necklace somehow had ended up coming into the possession of the Syndicate and we never hear about what happened to the Foxtons after the fact.
Sergeant - Another character from the Lord of the Clans novel. He was the man who taught Thrall the concept of mercy and honorable combat. He was also one of the few people nice to Thrall during his capitvity. When Thrall stormed Durnholde to break the internment camp system, he allowed Sergeant and his men to go free. We never got any further story for him after Lord of the Clans.
Calia Menethilās husband and child - In the Before the Storm Novel, we find out that Calia Menethil eloped with a footman from Lordaeron and they had a child together. After the third war, they had lived in Southshore up until Cataclysm when the Forsaken had blighted the town at which point they were separated. We never find out what happened to them after that.
Isiden Perenolde - The nephew of Aiden Perenolde. Last we heard of him, he went to Gilneas to protest who the leader of Alterac should be in the Beyond the Dark Portal novel. He has had no further mentions or appearances whatsoever.
Hath - A character from the Tides of Darkness novel. He was the commander of Alteracās military forces in the second war. He went against Perenoldeās order of allowing the Horde to pass through Alterac and remained loyal to the Alliance of Lordaeron. He makes no more appearances after this.
What long forgotten characters do you want an update on or to want to see return to the story?
EDIT: Added more characters to the OP that were brought up in this thread and some more that I thought of.
