Whatever it is... I think it's linked to this tweet he made about 25 man raiding:
"We have an idea, but it's a bit controversial so we're not ready to spring it on you yet."
Whatever solution Blizzard feel they have for the problem of lack of people wanting to raid 25 man... it's what he meant with this latest tweet. So what could help the problem of 25 man... cross realm guilds might be one idea, although does that guarantee the problem goes away? Debatable.
Personally I always live in hope that they merge US & EU battle.net so EU players could hop over to US servers, similar to GW2. But I doubt i'll ever see that in WoW.