How likely is it to have a scenario where both Horde and Alliance are dropped off in a map, where they both have to either assist an organization (like the illidari or some third party organization that is neutral) or just compete with one another to beat a threat or destroy an enemy faction?
For instance. Alliance and Horde are dropped off on an island, to assist Booty Bay trade merchants in capturing a regional pirate holdout. Both sides either start in the same place or start in different parts of the map and have to battle waves of pirates and capture nodes so that they can gain access to the center of the map (which can be locked by a gate) now this scenario can pit both Horde and Alliance against a PVE threat and have both sides work together and share resources, or have them compete (without killing eachother or attacking one another) to defeat this threat.
There can be mechanisms in place to allow for limited pvp around the map, like having players sign up and be enlisted into black ops teams and participate in unsanctioned missions that pits black ops agents of both factions against eachother in different parts of the map. the result of that pvp might not affect the outcome of the match but it might award only the players who participated in the black ops missions.
there can even be a lose conditions that trigger through mechanisms such as reinforcement counters and VIP NPCs. so the AI controlled side can win by either having the players die too many times and thus running the reinforcement counter to 0 (which goes down by 1 everytime a player dies and respawns) or have the VIP NPC killed when players fail to destroy an advancing wave that gets dangerously close to the heart of their base.
I think it can be a great way for Blizzard to bring some healthy competition into the game without having both factions openly fight eachother in an all-out war. it would make sense from a lore stand-point too as neither side is actively attacking one another but working together (or competing with eachother) to beat an opponent in a warfront-like scenario. and as for the black ops part of the scenario, well lore-wise both sides can deny liability as they are black ops teams on unsanctioned missions.