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Most of the races need to be divided further, not just lumped in under whatever their faction banner and assigned a leader to make MUHR FACSHUN WARH happen, so:
Humans:
This includes “standard” Humans, Kul Tiras and the Worgen.
Anduin Wrynn continues to lead Stormwind in his boring peaceful ways, ruling the single largest Human kingdom with enormous resources but a reluctance to wield them because of his passivity.
However, Lordaeron falls under the dominion of Danath Trollbane, splitting the Human factions with Stromgarde as capital of the New Lordaeron. This faction is weaker but more aggressive, making far more use of its advantages and seeking expansion at the cost of others.
Genn Greymane in Gilneas remains both a staunch ally of Anduin and his link to the New Lordaeron, showing how far he’s developed from his WarCraft 2 days of isolation and selfishness.
Kul Tiras remains under Jaina’s leadership. She’s hit a good balance at this point in time.
Dwarves
Including Dark Irons, Wildhammers.
Muradin asserts his dominion over Ironforge, forcing Moira back to merely ruling the Dark Irons. The child-prince Dagran may or may not inherit both thrones, and the Council of Three Hammers remains in existence but is loosened considerably. Strong ties to Stormwind hold both Ironforge and Shadowforge in check.
Falstad returns to Aerie Peak with the New Lordaeron faction. Attempts to retake Grim Batol and the Twilight Highlands are the primary concern for the Wildhammer.
Gnomes
Mekkatorque remains King of Gnomes, as even incapacitated he wins reelection. He is eventually resuscitated with the help of our new friends from Mechagon.
Prince - now King - Erazmin remains in control of the Mechagonian Gnomes, and the principal conflict between the gnomes of Mechagon and Gnomeregan is philosophical.
Night Elves
Tyrande remains in control of the bulk of Kaldorei society, pulling it increasingly away from the Alliance based in Stormwind. Malfurion and Maiev remain at her side.
Shandris Feathermoon, increasingly concerned with the Kaldorei’s darker path and having forged strong bonds with the Alliance, leads a splinter group that retains closer ties. Nominally subject to Tyrande, but functionally separate with their own goals and motives.
Draenei
Including Lightforged and Broken.
The Draenei largely remain as they are, maintaining close friendships to the Night Elves and Alliance both, and stay led by Velen. The Broken under Nobundo remain fiercely loyal to their ancient leader.
Yrel is brought back to the Draenei to lead the alternative faction. Less concerned with coexistence and finding that action, more than words, gain results, they align themselves more towards the aggressive factions of Night Elves and New Lordaeron. Some Broken, led by either Akama or Hatuun, Chieftain of the Argus Broken, side with them, as neither have much loyalty to Velen and both have a much darker aspect than most Draenei. This group may or may not include Lightbound Orcs.
Turalyon, Fareeya and the Lightforged form a bridge between the two extremes. On one hand, the Lightforged are zealous and quite violent, on the other Turalyon has a strong hand and Velen’s influence is tremendous.
High Elves
Including Blood Elves, High Elves and Void Elves.
By and large, High Elves and Void Elves remain as they are, given that they’ve pretty decisively chosen sides and have strong ties to their current leaders. Alleria and Valeera have a lot to do in Quel’Thalas and Umbric ensures the Void Elves largely have a strong tie with the Stormwind Alliance. This story can move forward without a need for separatism. Blood Elves, on the other hand…
Look, Lor’themar is just a bad racial leader. He’s a weak usurper with no purpose or point, and frankly he isn’t going to push the Blood Elf narrative forward. He’s the status quo personified and needs to go.
Rommath, the corrupting force at the heart of Silvermoon, likely takes the leadership from Lor’themar. This faction returns to the Burning Crusade roots of the Blood Elves as amoral, dishonorable and greedy. They either cling to Silvermoon desperately in the face of outside pressures, or they abandon it entirely to instead form their new base of power further away, possibly even in Outland itself.
Meanwhile, Halduron Brightwing leads the divergent faction. He’s always been shown to have more desire to push the Blood Elves towards unification with the High Elves, and thus the New Lordaeron faction. He may succeed in seizing Silvermoon from Rommath, or he may establish his faction further south.
Lady Liadrin leads the third, middle-ground faction. On one hand, the Sunwell and her ties to the Silver Hand lean her towards the New Lordaeron faction. On the other hand, her militancy and loyalty to Silvermoon lean her towards Rommath’s faction. She is a third option for holding Silvermoon itself, keeping the racial capital out of the line of fire between the other two leaders.
Pandaren
Huojin and Tushui included.
No change necessary. They are where they need to be.
Orcs
WoD-Mag’har, Outland Mag’har, Blackrock, Warsong, and more included.
Thrall returns to lead the Orcish Horde as best he is able. He is, after all, the Orc Messiah, returned to find his people lacking and in desperate need of his guidance not to stray again to violence.
Of course, no messianic figure is unchallenged. Geya’rah, Thrall’s counterpart from another timeline, brought into the Horde in a time of war and raised on propaganda, she is strong and vibrant, young, eager - Garrosh reborn, in truth.
This, I feel, more than any other, is the pivotal change I would want in the faction leaderships: No Mak’gora. Geya’rah and Thrall both lead Orcs, but it is down to each Orc who they follow. The ways of honor and redemption, peace and stabilty under Thrall - or the ways of glory and carnage, war and strength under Geya’rah.
Split the Horde down the middle, on one side Geya’rah’s “Iron Horde” pursuing vendettas and bloody-minded conquest, and on the other Thrall’s “New Horde”, making peace with Stormwind through Jaina and seeking peaceful coexistence with their neighbors and the natural world while tearing out corruption in their own ranks.
Tauren
Highmountain, Grimtotem and Taunka included.
By and large the Tauren remain as they are. Baine and Mayla are both good leaders for their respective peoples. They are concerned with the pursuit of peace and lasting stability, and can ensure it. They remain with Thrall’s faction of the Horde, the Tauren ideal of peaceful lives on the open plains of Mulgore the most important thing to protect.
Magatha, her Grimtotem, and the Taunka of Northrend on the other hand feel like the best way to allow the Tauren to divide. The Grimtotem and the Taunka are not like their Bloodhoof and Highmountain cousins - they are savage. They are strong. They side with Geya’rah and make their home in harsh lands where lesser Tauren and their weak allies do not survive.
Trolls
Rokhan and Master Gadrin face a similar split to the other core races of the Horde, but they have no bad blood between them, so the split is more philosophical.
Master Gadrin remains in the Echo Isles to peacefully build up the Darkspears, Rokhan endeavours to strike out with those who follow him to join Geya’rah and make the Darkspears known as more than just a village of fishermen feeding Orgrimmar.
Goblins
Gallywix dies very suddenly when his balcony collapses under his fat bulk after a renovation. Gazlowe, who definitely didn’t have anything to do with the faulty renovation job, takes over the bulk of Horde Goblins as he’s already more or less the man for the job and his ties to Thrall are strong.
However, dissent spreads quickly - after all, Gazlowe might be a nice guy, but has he got the ears for being Trade Prince? He’s not even Bilgewater, he’s a Steamwheedle! No, that won’t do. Fortunately, the greatest mind of the Bilgewater Cartel, the saviour of the Lost Isles refugees, Sassy Hardwrench rises to the occasion and boots Gazlowe and his cronies out of Bilgewater Harbor!
Divided by Durotar, Gazlowe and Sassy take aim at different goals for the future of Goblin profits. Gazlowe, of course, keeps his close ties to Thrall and the New Horde, while Sassy reaches for the “Iron Horde” with its greater threat - after all, “The riskier the road, the greater the profit.” (Rule of Acquisition #62)
Undead
The Forsaken are in dire straits, and Calia Menethil seems to offer the best hope for a new future. She is their Queen, by right, and she can restore Lordaeron to its lost glory, most assuredly. As Calia takes the throne of Lordaeron once more - in absetia the actual throne, as the old city will probably take time to be cleaned up - and forms strong bonds with the Alliance to the south, however, many - if not most - Forsaken find her a change too radical from what their new lives have been.
Lilian Voss becomes the rallying point for the ‘true’ Forsaken. Those whose loyalties to the Horde are strongly built, those who reject their humanity, those who still fear the consequences of attempting life anew. Threatened by the oncoming wave of New Lordaeron and losing the support of Thrall’s Horde, they side with Geya’rah and entrench themselves in Northrend and the northern corners of Lordaeron.
Nightborne
While Thalyssra remains the nominal leader of the Nightborne in Suramar, it’s becoming clear that siding with the Horde was premature and only brought more death for her people at little gain. In future, her focus is the peaceful rebuilding of Suramar, as an independent state, answering not to Horde or Alliance.
Others see the benefits of expansion, most importantly Valtrois, who has already gained prestige fighting as a champion of the Honorbound. Under her direction, one faction of Nightborne continue to support the Horde - the Iron Horde faction of Geya’rah - in the hopes of reaping the benefits.
Which side proves the more successful remains to be seen, as does whether or not another Nightborne faction will rise to seek reunification with the rising power of the Kaldorei.
Zandalari Trolls
Talanji is fine. She just needs to keep the Zandalari Empire independent and not fall in line behind any of the other factions, instead siding only with those who her people will benefit from.