First of all i'll mention that I don't know jack shit about WoW lore, so in that regard i'm not sure if this could be implemented or not. But I have heard some names like Gul'dan, Grommash and a couple others that are considered to be viable prospects for an end of the expansion boss, but so far nothing has been announced.
I mention the idea of a council fight because it hasn't been done in the past; and it feels like Lich King was the pinnacle of what Blizzard can hope to achieve from an expansion ending boss encounter. Both Deathwing and Garrosh have paled in comparison regarding encounter design (which can be remedied) and lore (which can't be remedied). It seems like the only boss that will give players that loregasm feeling of killing an end boss is Sargeras.
So perhaps a council fight will offer something new and exciting as an end boss? The possibilities for encounter design are increased significantly and adding 4+ phases to a council fight could make it really intense.
Will you be satisfied if the last boss of WoD is a council fight?