Regional servers merge (EU, US).
EDIT: I'm not sugesting playing from EU on US server and vice versa, you would play on your region's server only, but that would function as one large server with autobalancing phases to avoid over/under population.
Think about it, we allready have CRZs and cross server RealID chats, so the tech behind it is allready in stages of live testing.
The way it will function is you log in to game on youre regional server, EU for example, and youre character gets logged in on one of the physical servers allready in existance. Server which you get logged in is choosen based on current player/faction balance on active servers, and functions only as one phasing of regional server, to avoid overclutter in zones. Regardless of ''server phase'' you're in, at any given time you have full acces to youre guilds chat, and AH is unified for all phased servers.
Major chats (city) are at risk of beeing overcluttered if unified, so one of two solutions will be implemented:
1- There will be tabs (city1, city2, city3...) to switch between different phases chat.
2- There will be /1Trade, /2raid, /3dungeon, /4arena, /4RBG, /5bg, /6guildrecruitment... regional chats. This is in theory better option, but due to nature of community it would all end up in /trade so it might not work.
When you group with someone you all get phased in raid/group leaders phase.
That's it. No more low pop servers, no more imbalanced servers, no more server xfers to find a raiding guild.
Of course, since blizzard makes 20E per server xfer i fully expect faction change to go from 25E to 45E to compensate. They will call it ''Turncloak Tax'' and argue that with the increase in war intensity between Horde and Alliance changing sides gets ever more difficult and expensive endevour