I think that the Warcheif/High King roles being vacated (or not re-claimed, in the Horde’s case), would work well as an act of de-escalation, on both sides.
In the case of the Horde, I like the idea of a council, coupled with the further character cusomisation options that’s coming. It would allow for more depth in the races of the Horde, and actually show which (for example) of the Troll, Orc and Tauren clans actually belong to the Horde, allowing for both characters and NPCs to show individual identity within not just the horde, but within their own race.
In the case of the Mag’har, in particular, we could see some character growth through interactions between MU orcs and Mag’har of the same clan.
On the flip-side, it could make for a very crowded council, and so it’s practical use for decision making (how the Horde acts as a whole) could become a nightmare. Not sure how this would then be solved.
The Alliance is much simpler, most of their races are essentially single tribes, and so representation becomes easier. The only tribes not represented (that I can think of) as a primary or allied race, are the Wild Hammer Dwarves and Stromgarde Humans, which can be solved with character customisations (like it or not, Dalaran is now just a city and not a nation, which is home to a neutral organisation made up of both Horde and Alliance representation).
It could even be an opportune time to re-introduce the people of Alterac (given that some on this forum seem to want to overturn that particular stone). The forgotten nephew, Isiden Perenolde, returns. But not to reclaim his kingdom, rather to encourage those of his people that either fled to other nations, or renounced the Syndicate, to step out of the shadows and re-join the Alliance. It gives Blizz another future villain-bat option, or simply for the sake of adding to the lore/story (crazy, I know).