Cross Realm Auction houses are the most plausible theory IMO. It addresses one of the big economical issues with CRZs where low pop realms without an army of botters can't even get mid level crafting mats from the AH. And they seem desperate to get people to stop hating them so much so something that makes them a bit less objectionable is probably high on their list of things to add.
That's why server mergers are highly unlikely, yet. As we saw with the numerous iterations of them trying to make meeting stones a useful LFG tool, the most obvious and simple approach is often something they like to put off for a few years while they try to make a more convoluted solution work.
The one I'd most like to see though is the content scaling idea. Either scaling it up for level 90s or scaling players down to appropriate levels. Also, a LFG-like queuing/loot system for old content to make transmog and achievement runs a bit easier so there's less people needling guildies to do runs with them or resorting to PuGs which usually just turn into massive whinefests.