Basicly right now the retail WoW can be played as time-intensively and on whatever difficulty you want. There is always something to do, however you hit diminishing returns quite soon. There is tons of hard, engaging content to be done with friends.
I think Classic is kinda fad, something a lot of people think they wanted, but after some time they will realize they did not. Of course, I might be wrong in which case I am happy they got a completely new game to play and hope they enjoy it as much as I enjoyed the two years of WoW Legion(had to stop, because I did not have time to play at the level I wanted).
On the other hand, I can't wait for Warcraft 3 Remastered, that will work by default with most of old custom maps. I am taking a week off for that. :)