With the news of a possible "Classic +" as well as an "inevitable" Classic WotLK down the road, I hope they eventually link Classic WoW accounts with Retail Accounts.
For clarity, when I say "link", I don't mean characters exist in both accounts /versions. However I would like to see them bridge the "Achievements" as well as the "Collectables" chunks of both sections. It made sense not to do so when Classic initially launched, but I think that's no longer the case.
With Classic WotLK; achievements would come into the game anyways (since that is the expansion they were introduced). I think it would be nice to have a toggle in Classic to show achievements you've already gotten in retail, but then toggle it off to only show what's achieved in "classic". Rewards achieved for these achievements would be available for use in retail (i.e. mounts).
I think collections in general, Mounts, Transmogs, pets, toys, etc. that are obtained in classic should be usable in retail. Again, I think this made sense to not do on initial classic launch, but as "Classic" has become a thing of it's own (and will likely persist through many expansions to come), these should be bridged.
It would have to have limitations (i.e. If it is Classic WotLK you're playing, then you can't be riding around on mounts only obtainable AFTER that expansion. And infact there is probably an argument to be made that the bridge only flows one way. I.e. things you've gotten in retail you can't use in Classic (only vice versa). That way if you see someone riding around on Mimiron's head in Classic WotLK, you know they achieved it IN Classic WotLK and didnt just farm it on retail in outdated content.
This is purely built towards my taste in how I play the game and interested in if other people would want this as wll.