Would be really cool if they can link unique rewards you get in retail for achievements you do in classic versions. I think retail/live would best work as a collection/completionist game that should stop at the 10th expansion.
A WoW 2 then follows giving us something completely new and refreshing that is not linked to the previous 10 expansions in terms of player specific things, giving us a new vanilla experience with nothing carried over.
I used to be against this idea, but I just don’t see how WoW can survive against the competition if things continue as they are.
EDIT
it's not that hard, if they just look at my name and do what it says....plus on top of that all kinds of quality of life changes, the best of all of WoW, all in one put together on top of a remastered vanilla. They can even do a poll of all features in all of WoW history so far and include the most popular ones. Make this version of WoW, the WoW plus.....keep adding expansions to the normal WoW as usual and whatever is popular, add it to the PLUS