I have searched through discussion and some people ask if there will be possible to merge achievements between retail and classic or just have account-achievements between WOTLK characters.
Of course, officially none of that will be possible. Blizzard won’t merge both versions of the game nor change WOTLK achievements to make it account-bound.
Personally, I only play Classic versions to complete things on Classic that I never did back during TBC and WOTLK. I am not interested in repeating most of the things from Classic, nor playing dungeons, raids, and most achievements I already completed.
That is why I like the idea of having cross-achievements. Because that allow me to open the achievement tab in WOTLK and see what things I never done, no matter what version of the game.
Thanks to addons that is possible. Yes, add-ons can make it, because achievements are just information about what you have done. For example, Classic and Classic TBC do not have official achievements by Blizzard but there is an addon that adds Achievements to those versions.
I already make a private similar addon that I never release. Not for achievements but for quests. Basically, I make a Retail addon that export completed quests from the database of the Addon AllTheThings and another Classic addon that import that data and change the database of the Addon AllTheThings so it shows me as completed any quest completed on any version, that way I can focus on quests I never did.
That is why I believe it should be possible to have an addon that export achievements and another that imports achievements and changes UI to show those achievements as completed, even if Blizzard didn’t recognize that.
Even I think it would be cool to personalize the addon to import not all achievements from retail but only achievements from retail that were completed on the original WOTLK so that an achievement completed in 2008 could count as completed in Classic WOTLK but one completed in 2012 don’t, because maybe I could be interested in do that achievement the proper way, for example in a dungeon group instead of solo.
I suppose it even could go beyond that and do the same patch by patch, so before releasing Ulduar in Classic WOTLK not import any achievement that was not completed on the original patch 3.0, to be able to choose to do it during its proper phase.
I suppose the opposite it is also possible and shown in Retail as completed achievements from WOTLK that are no longer possible in Retail, but I am more interested in Retail → Classic WOTLK, even WOTLK → Classic WOTLK, not Classic WOTLK → Retail.