Cross-realm zones are and were extremely unpopular. It’s what has given cross-realm anything such a bad stigma.
There can be cross-realm groups, raids, and guilds without cross-realm zones. These cross-realm activities would go well with the modern auction house.
Having guests on another realm was bad in that people from high-pop realms invaded low-pop realms to raid the resources. This could be prevented similar to the inability to trade things cross-realm.
Trading between cross-realm can be done with raid loot similar to the grace period of trading the loot currently in the game. What about enchanting mats? Those mats could also receive the grace period. Or just bring back the disenchant roll, which would solve the issue of disenchanting for the group when someone from a different realm is present.