After years of people asking the wow team has finally announced they will be adding cross-faction instances to World of Warcraft. Starting with the 9.2.5 PTR they will test this option for premade groups for raids, dungeons, and rated PVP.
This will be completely optional by just checking a box when looking for a group or looking for people in premade group finder. This also will not be like mercenary mode for battlegrounds where you change factions and races. This is true cross-faction, you can be a human and group up with an orc.
Now with this said the blue post that was posted today did note that some legacy raids would not work with a cross-faction group. They all seemed to make sense like Battle of dazar'alor which heavily relies on the faction divide. ICC is another one that will not work because of the gunship battle, and Trial of the Crusader. Pretty much if the faction divide is the core piler of the content then it will take more time to get cross-faction play to work.
All this being said, this will not change anything in the open world of the game. So if you have war mode on the opposite faction is still game. Cross faction play is more for the end-user, not lore. The Horde and Alliance still have their uneasy armistice and the drums of war will once again thunder. But for you and me if we need an extra body for raid our friends on the opposite faction can help us out.
Maybe one day the Horde and Alliance will find true peace, for now, the faction divide is still a pillar of Warcraft but for gameplay sake, the developers will make a slite change of course and allow us to play together.