See this is the response I was looking for because I also agree that would be the card developers would pull to justify continuing the faction war. But they can keep the "war in Warcraft" and have peace between the Horde and Alliance. With the war ended, there would be no need for "war mode" anymore. That said, we can't remove a core aspect of WoW that has been around since its incarnation, that being world pvp. So instead, why not introduce two new factions? 'The Rebellion' and 'The Resistance', one for the Horde and the other for the Alliance.
Two groups formed out of their hatred for Horde/Alliance that oppose the peace between the two factions. Instead of enabling war mode, by enabling "Join the Rebellion!" or "Join the Resistance!" you go from Hostile to Exalted (reputation serves no purpose other than lore), become hostile to "opposing" faction players, and can interact with NPCs of your faction in their respective hubs that are now the locations you use to purchase PvP gear, pick up CTA quests, and redeem your weekly chest.
This simply becomes the new PvP mode and operates almost exactly like war mode. You're undercover of course, so the Horde/Alliance are none the wiser to your activities outside of major cities, but you may get a whisper from time to time from loyalists who give you their support.
The Horde/Alliance know of these factions of course, but do not cast blame upon each other for they expected rebellion. Perhaps certain events could occur in which the Horde/Alliance try to stop you, acting as new warfront or bounty quest lore.
Anyway, just a thought.