...I think we may be seeing the stage being set for the return of Horde racial factions. I feel like Sylvanas is a placeholder like Vol'Jin was in that regard and Varok Saurfang is obviously a placeholder for whoever the new Orcs' leader may be. Speaking of: Isn't Saurfang Warsong originally? I do not think it's that big of a stretch for AU Grom to be brought in to become the new leader of the Orcs of Azeroth.
The Lore could be written as while Sylvanas was busy on The Broken Isles, Saurfang secretly met with Grommash Hellscream on Draenor where he told him the rest of what happened after the vision Garrosh showed him. He tells him about Thrall, the events of the Third War, present-day Outland and the remnants of their people being discovered in Garadar and about Garrosh. Most importantly, he will tell him about his MU self. Finally, Saurfang asks Grom to do what his MU self and Garrosh couldn't and show the Horde that it is worthy of being graced with valor of Hellscream.
That aside, I always felt the reason some Clans are still mentioned separately from the Horde (mainly Warsong and Frostwolf) is because we may see a return of that style of leadership among the Orcs. Now, it feels necessary. Maybe even make it a quest chain where the Player joins Saurfang in recruiting Orcs to lead various Orc Clans to help bring order to their people on Azeroth.
The Alliance equivalent could revolve around Yrel being recruited by MU Velen to lead the Draenei on Azeroth. The player character joins her as she travels to meet the Alliance Faction leaders, learning more about the Alliance than she knew while on Draenor. The chain ends back at the Exodar where Velen reveals he is going to personally train King Anduin Wrynn full time and she will leader their people in his absence. Bolstered by The Council Exarchs, she will be able to handle the responsbility like she did when his AU self passed on his power to her.