In a Warcraft Wiki Q&A with Associate Game Director Morgan Day, we learned that Cross-Realm Guilds are coming in The War Within Expansion for WoW.
Additionally, the restrictions on Cross-Realm Mythic have been eliminated allowing for players to enter Mythic Raid starting on day 1 of each new Season.
Cross-Faction raiding was allowed in Shadowlands Patch 9.2.5 and then Cross-Faction guilds became a feature in Dragonflight Patch 10.1. This next step will continue to break down barriers for players allowing them to create cross-server communities to raid with that aren't content-restricted allowing for greater flexibility when choosing servers, factions, and guilds.
Be sure to check out the full Q&A Notes on Warcraft Wiki for more information on The War Within Expansion, Warcraft Rumble, and Warcraft Classic.