I’ve been thinking about this since Ion asked this. The honest answer is “I’m not sure”.
What I do know is that no way in hell do I want to end up in WoD again (though honestly, the raids were a lot of fun - the only fun part), nor would I particularly relish doing cata again.
MoP was a lot of fun, and I enjoy that, but after that… no - just no.
I also do not trust the current batch of devs to be good judges of quality content, so I don’t trust them to make many changes and additions - hell these are the people who screwed the game up the first time and thought it was a great idea, and letting them do it again would be heartbreaking.
I suppose what I would like to see is to modify the game to allow the players to be able to set the patch number they wish to play. For example, by setting the game to patch 1.phase2 you could go and experience the game as it was in vanilla phase 2 (complete with zones and character items & abilities as they were), be able to raid BWL with only items acquired in patch 1.phase2 or earlier and with other people with the same restriction. Then after that raid, be able to switch to patch 3 phase X and raid ICC with a different group. This would require creating a new system of item, gear and talent versioning and management - one that keeps track of what patch & phase you acquired items and prevents your bags from exploding. There would be many other details to hammer out - like the AH and world PVP - but I’m sure the problems are not insurmountable.
The allure of such a system would be we could play whatever content we want as it was when it was current, regardless of current patch/phase. It could even allow merging with retail if handled right. Spreading a dwindling player base across more and more barely alive servers with different versions of the game running is not long term viable. Since content could be played as if it was current, there would be no reason to not allow people to farm for the old items that became removed from the game at their own pace. Since all of the game’s old content would be available, the amount of stuff to do would be near limitless. Heck - you could even create a “new server” experience periodically by allowing character creation on specific special patches without creating new servers, and these special patches could be merged back into normal patches seamlessly after some time.
Adding some kind of in game realm leaderboards for achievements - like dps on bosses, most achievement points on your server, speed run times, etc, would keep people playing too (heck - even just a list of heroes who cleared the harder content any specific week).
I think some of the biggest problems with Cata involved the permanent changing of the of world (and removing a ton of stuff), and the changing of the talent trees to be much more pruned down (it’s not like ability pruning in WoD went down any better). If there was a bronze dragonflight NPC in zones allowing people to travel back, and the talents did not make you feel like you lost flexibility in your toons, I think the xpac would have been much more warmly received. What I propose would solve both these issues.
Do I expect to do this? No.
Depending on how fun things are, I might stick around in the various era realms for a bit… but ultimately that’ll probably be the end of the road.