Blizzard. Why not just honestly copy FFXIV as a solution here.
Make the realms all connected in some way technologically, and use a hub for the player in game to “visit” other realms and even trade some items or small quantities of gold and such.
I think people love the idea of their own server and sort of closed off community, but in practice most people just want to play on a “mega” server with many people to choose from.
Allowing an in-game hub where players can visit other realms might alleviate some of the stress that forces players to huddle together into mega-servers, and it wouldn’t be annoying and uncontrollable like phasing on retail realms can be.
They would have their character name and the server name attached to it when visiting another realm, but they would always belong to their home realm.
They wouldn’t be able to join a guild that wasn’t on their home realm sure, but like I said above, maybe allow small quantities of gold, or items to be traded among visiting players and home turf players. They could group, quest together, raid, and socialize etc etc.
FFXIV really has it down so well and really should be the standard for realms in all MMOs. This solution should be on retail realms in my opinion. Phasing is way too unpredictable and doesn’t feel good when the player can’t control their phase.
(I’m also aware that there are probably ways the players might abuse or exploit this if it was implemented, but those kinks could be straightened out, and I feel like this solution is worth the effort.)