I think at this point it might be a logical step if the horde would split.
In the beginning it consisted of Orcs, Tauren and Darkspeer Trolls - sharing the concept of a union based on honor and the fight against the threats of Kalimdor.
When the forsaken joined, it was already an alien concept and mainly happened due to them beeing rejected by the alliance and to weak to stand alone. They never really shared the core values of the original horde and broke the concept of honor in multiple occasions. Bloodelves joining in wasn't really fitting either.. they couldnt care less about honor and instead strive for power, by methods of questionable moral if needed.
I think at this point with so many races joining in over the years there is no clear core unity atleast for the horde faction anymore. At some point a federation might just be to big and to disparate to go on. And atleast the horde seems to be at this point. Also it would provide a reasonable continuation for Windrunners storyline without the bland "she will be just another Garrosh" thing. It would also completly flip a core design of the game and allow for very new and interesting constellations. Player groups and guilds beeing ripped apart in the process might be a bit harsh.. but then again.. this is what would happen in a "civil war" after all.
So i think it would be logical for the Horde to split into to factions:
The Horde (Orcs, Tauren)
New Faction (Undead, Bloodelves)
.. with various splits for the rest of the races (Darkspeer / Zandalarian Trolls for either Faction) or restrictions (Pandaren only available to Horde).